Although the Unbreakable Vow spell is done voluntarily, it is possible that it results in one's death.
Based on that - is the spell legal in the magical world? Consciously endangering one's life is considered a crime no matter whether the person agreed with it or not.
For example if you make a bet with someone, that he/she can dodge an arrow that you fire, you will sill be charged at least with attempted murder (or actual murder).
Wizards and Witches seem to be more cavalier with the ideas of 'personal safety' and 'lives' than Muggles (see: Love Potions being legal, despite the fact that Muggles would consider them... uh, certainly not granting 'consent'). An Unbreakable Vow seems pretty impossible to actually force someone to do- i.e. if someone knowingly makes an Unbreakable Vow then breaks it, it's entirely their fault.
Obviously I imagine (but there is no evidence) that tricking someone into making an Unbreakable Vow that you know is impossible to complete will probably still constitute murder, just as the act of casting Stupefy isn't illegal, but you can still kill someone with it if you hit them off a broom or something (as stated by Harry).
It's entirely possible that the Vow won't even work if the task is actually impossible to complete.
That said it's entirely possible it's not a thing ordinary wizards just do. Clearly marriages would be a lot more stable if the wedding vows were Unbreakable, for instance ("I will never cheat on you because if I did I would literally drop dead.").
So why waste time legislating it? If someone wants to risk their life it's their business. If you don't want to make the Vow, just don't make it.
