Why would anyone care one way or the other? Why force the players to do something that makes absolutely no difference whatsoever?
and btw what leagues are you talking about, usually players just took them in to look a little classy and clean. theyre shirts usually end up untucked during the game anyways.
it's easier to call shirt pulls and stuff like that when it's tucked in (plus it's neater). MLS starts with their shirts in, and end up with them out, as do most other leagues
I pick up yellow cards all the time because refs make me tuck my shirt in and i just pull them out. people cant really grab hold of mine because its tight on me.
if you have a shirt that isn't skin tight and it's not tucked in then a defender could easily pull on your shirt and the ref won't even know the difference. If ur shirt is tucked in and the ref thinks he sees something and your shirt comes untucked u have a better chance of getting a call. also if u get pulled down u can have a good time argueing to the ref that ur shirt was tucked in