First - I don't agree with filling rosters after a position is secured, or "sand-bagging", or many of the other questionable rostering practices. That being said, I think you have to be very careful when making the argument that "no other sport" allows crossovers the way the cheerleading industry does.
Cheerleading is NOT any other sport.
Many other sports have multiple games and tournaments to determine an approximate ranking in relation to the other teams. The "best" team in a league is usually determined by competition between all teams in the league, perhaps with a few championship rounds between the top teams in the league. Team A usually plays Team B several times throughout the season before it is determined which team is "better" than the other.
Results from competitions in other sports are generally absolute (yes, there are exceptions, but...). There is a final score, or a winning time, or something that is easily compared to other competitions or other teams or other athletes.
Many other sports have a much smaller continuum of skills required - you expect a 9-year-old soccer player to pass the ball, but you don't expect a 19-year-old to take the ball down the field while walking on his hands. (I know this is absurd, but compare it to the difference between stepping up 18 inches to stand on someone's thigh and stepping up over 6 feet to stand on someone's extended hands and contorting your body).
Other sports aren't judged on whether a majority of the team can perform a particular skill. (Hypothetically) If they can score goals while only 50% of their players know how to skate on ice, nobody takes away their points.
I know there are safety reasons, practicality issues, etc. that make the cheerleading industry so different and unique and I'm not criticizing that. I just want to point out that we can't compare everything that happens in cheerleading to things that happen in other sports.