I am former Army. Its not a mentality thing as much as its a professional courtesy. On the Army Guard side of things, we used to have new LT's that would hurt us after deciding they wanted a different assignment once done with OCS. I get it, preferences and perspectives change, but when you commit to a unit and get them slotted into a specific position, only to have them renege really throws off the balance of the manning and workload for 60+ Soldiers. I am extremely happy to hear you say its not as big as a problem in the Air Force. When you have a Company of 60 Soldiers and a 2LT acting as the Commander because there is a deficiency of Officers in the state, every person that decides to change their mind has a big impact. Thats a whole 'nother rabbit hole though, again, glad to hear its not that way in the AF.
Its my opinion that when you commit to a UPT slot and work with that unit for the better part of a year and enlist, thats where you stay. Barring any major family issues that would require you to be near your home, I can't sympathize with any decision to leave. Saying "no thanks" to a unit after a few weeks of getting a better offer and not doing any in-processing, I can totally get that. This is a job of a lifetime. Do you want it or do you want it with stipulations?