Bat rolling is a process primarily used on composite bats. Composite bats get "hotter" with use. They can be broken in naturally, ie, batting practice. Or they can be rolled (accelerated break-in, ABI) through industrial rollers.
This practice is very common, and controversial, in adult slowpitch, where composites rule.
On softball bats, at least, a moderate rolling more or less reproduces natural break-in; a heavy rolling significantly increases the size of the sweet spot and trampoline effect.
There are rumblings on mess. boards that "rolling" is becoming a factor in youth travel and all-stars.
This is the first I've heard of it about HS baseball.
In softball, as of recently, HS and College bats will have to be rolled before they are tested by the ASA.
I don't know if composite BESR bats have to be rolled prior to testing.
This would be a qood question for the tester, Jim Sherwood at Umass/Lowell.
If they adopt a policy of rolling composite BESR bats before testing, then we won't have to waste years debating the ethics of the process on this board.