SMDDAD: I agree that they don't throw every ball back at full speed, but the good ones are snapping back to the pitcher, trying to get a baserunner off balance. Or in my daughter's case she is throwing that hard from her knees, therefore she is using all arm and core (she is 15 and has a 1.82 POP from her knees)....
I would be concerned for her shoulder, because she's throwing from her knees. My DD has the same POP time throwing down from a runners on position. She, too, just turned 15.
These are pretty amazing times. My DD #1 played in a couple of tournaments a couple of years back with a catcher who was 15 at the time, with a pop time a little over 2 seconds. She was 2.0 earlier this year, so probably a little slower back then. I can certainly remember the chilling effect it had on the other teams' base runners. A lot of teams started out stealing aggressively, then stopped altogether when they saw what she could do.
That girl's father told me a pop time of 1.8 was what the colleges were looking for. I can imagine, if your girls have around 1.8 at 15, the colleges will be noticing already.
When my DD #1 played, she almost always stole on the first pitch. It was extremely rare for her to find a catcher who could throw her out. I remember seeing a catcher throw her out in the first game of a double header, so DD made sure to steal successfully in the second game. However, DD #1 never faced a catcher with a pop time under 2 seconds.