I'll play or at least try to explain the reaction we observe. Can't speak for everyone, but I think most (maybe all) would be absolutely fine with the scenario you lay out above. I think your real question is why do people react negatively when our players (especially starters/stars) hit the portal, but welcome with open arms players that transfer in. The latter doesn't need a lot of explanation. I'll try to dissect the former.
I think the initial visceral reaction when "our players" choose to leave is deeply-rooted in the fragile psyche of the Mean Green fan base. Those here are passionate about the University of North Texas, want good things for the school, want the athletic teams to be successful and desperately want the recognition that comes with winning and championships. In the case of athletic success much of that is dependent on the athletes. They become the vehicle for that recognition.
When athletes choose to come to UNT over other schools it serves as a type of consensual validation of the choice each of us made to attend here. When those same player spend 3 or 4 years developing their skills, becoming starters or stars on the team, they are seen, rightly or wrongly, as a catalyst that will surely get us over that hump and the recognition we are all starved for. Plus we get to know them well...they are one of us. Then on the cusp of everything good that will come next season, they choose to leave. It's like they are saying they have outgrown us and they cannot get where they want to be if they stay associated with UNT. For those that are so passionate about North Texas, that's hard to hear and it hurts. On top of that, throw in the realization that any success that athlete may have in his or her one year at XYZ U will accrue to XYZ U and not North Texas. That's like rubbing salt in the wound.
Eventually, I think everyone gets over it, but for the long-suffering fan base it's just another hit. It's a rather ordinary and understandable emotional reaction when you think about it.