After watching 'Meat Swap' I was lead to believe that all witchcraft in Supernatural seems to work by worshipping some sort of demon, though I'm interested in if this all is in fact a most.... I had a moment of fridge logic and realised we see a few instances of Bobby (in the Witness episode) and Castiel (when he searches for Anna) performing magic.
How do they get around the 'devil worship'?
Obviously, based on you even asking the question, it must be true that not all witchcraft in Supernatural requires demonic worship.
In fact, magic itself in the series is just one more mystical tool available for those that know how to use it. Much like the various weapons can be wielded by anyone, even if they are not its intended wielder (e.g. anyone could kill a demon with Ruby's knife, not just demons), anyone can wield magic if they know how to do it.
However, not all magic appears equal in the Supernatural world; if we take a look at the kinds of magical events that happen, we can group them into a few categories. As @Thaddeus points out in the comments, the show itself is not known for being rigorous in its depiction and classification of magic; there are more commonly-accepted terms for the types of magical effects that we see on-screen but the characters rarely seem to use them. The following is mostly a descriptive, rather than official, breakdown of the kinds of magic that happens in the show.
This is most of the magic we see early on in the series; humans who have made deals with a demon in exchange for access to magical powers. Note that these people are able to produce varying magical effects simply by willing it to happen, but they also employ traditional spells, as well as totems and charms imbued with permanent effects.
It appears that this kind of magic does require supernatural intervention for a mortal to achieve, typically demonic intervention, though I supposed it would be possible for some other powerful being to give a mortal access to this kind of power. The items that witches create, however, appear to be usable by anyone (the boys steal hex bags from witches and carry them around, for example.)
This is my own term for this type of magic; in the show, it is what the terms "magic" and "spell" mean when there's no context to indicate otherwise. These are the magical practices that that involve ingredients and incantations that anyone can apparently use, without any need for association with demons.
In this type of magic, the effect comes from just "following the recipe", so to speak. Anyone capable of recreating the exact conditions needed by the spell can make it happen. However, they are limited by the precise constraints of the spell, and we've never seen any indication that a non-witch can "create" (or possibly "discover") a new spell on their own. The magic here is "the knowledge". Just by knowing the list of ingredients and symbols that Balthazar used to send Sam and Dean to the "real world", Dean fully expected to be able to recreate the effect on his own. This form of magic simply is -- its part of the world that is available to anyone who knows. (And is apparently not part of the "real world" since it didn't work for Dean as expected.)
The incantations used in this type of magic sounds like it's supposed to be Latin (though modern TV almost never gets classical Latin pronunciation right -- it's always Chuch Latin at best). Even the exorcisms that Sam and Dean occasionally perform can be lumped under this type of magic; arguably that "power" may be more divine than mechanical but the show is never clear either way.
Much of the magical effects we see in the later seasons is Enochian magic -- the language of heaven. The sigils that the Winchesters use to hide from, banish, or trap angels and demons are largely Enochian. (If I had to guess I would assume a Devil's trap is probably this type of magic, though I don't think it's ever stated.)
Also, some of the incantations that angels like Castiel use are Enochian, which in the series has a distinctly different sound than Latin. (Side note, Enochian is a "real" language, in the sense that the Supernatural writers didn't invent it, but borrowed it from the guys who did.) Again, as the language of Heaven, being able to speak it allows one to produce divine effects that would be well beyond the range of mortals.
There seems, IMO, to be distinction between this and the "mechanical" type of magic, in that "the magic" here is the language itself. This angelic magic derives its power directly from the words (written or spoken), as if saying or writing something in Enochian makes it happen. Note, however, that one does not have to be an angel to use written Enochian sigil magic. I don't remember if we ever see a mortal (or demon) use a spoken Enochian incantation (if anyone else does, please remind me! )
And, of course, the non-mortal creatures in the series all have their own abilitys and powers that can occasionally seem like magic. These creatures also seem capable of using more mundane magical forms when it suits their needs. Castiel, for example, uses a normal spell to locate Anna, presumably because his own innate abilities were being blocked or otherwise unusable.
No, not all magic is given by worship of demons - not even most of it.
Rowena explained that there are 3 types of witches: borrowers, the Naturals, and the Students. Borrowers are as it described, people who have obtained their power through demonic forces. The naturals are people who have obtained their power from biological development. The students are people who have obtained their power through practice and training.
-1because your question saysmost witchcraftrather thanall witchcraft, thereby answering it yourself.