Who said they weren't?
In TNG era we see several instances of cadets being disciplined for various actions, notably the flight accident when a cadet was killed. So, we know that discipline does probably exist.
Regarding this specific incident, when Pike entered the bar, he ordered all the cadets outside; what is to say their details weren't recorded and they were to be given discipline later on? We simply don't have sufficient evidence to believe they weren't disciplined in some way.
Yes, we do see them sitting on the same shuttle as Kirk. However, bear in mind that it's possible they were to be disciplined at Starfleet Academy later; they weren't heading straight off to serve aboard the Enterprise necessarily - Kirk ends up at Starfleet Academy from that trip. So, I'd argue that they probably did receive discipline of some form, but it was to be dealt when they arrived at Starfleet Academy - all instances of cadet discipline we see in the rest of the Star Trek (that I can recall anyway) are conducted at Starfleet Academy!
Yes, by the time that Kirk is aboard the Enterprise the other thugs that were involved in the brawl are also aboard, this is several years later. In fact, based upon this, we could draw the conclusion that they were disciplined! If you consider that they were cadets at the time when they assaulted Kirk, then three years later were still cadets, probably in their final year (because they were serving aboard a starship), then, yes it is possible they weren't disciplined. However, if they were in second year or above, then being held back could relate to disciplinary action being served. Just for context, in the Nova Shuttle incident we see in 'The First Duty', for manslaughter, Wesley Crusher was required to repeat that year at the Academy. If a similar punishment was dealt to the thugs involved in the brawl, this could explain this. However, I do concede a more plausible explanation would be that they were simply first years.
So, basically, I fail to see any evidence to support the contention that they didn't get disciplined! Even if it was relatively mild disciplinary action, I expect that, considering how abhorrent violence was considered in 23rd century society to a Starfleet officer (well, except for Kirk in the prime reality), they would have been dealt at least some disciplinary action!