I think it's because if somene pull out harder the lush, over time it can get broke, I tell you from my own experience, my last lush lasted me only a year. I don't hear or read about it's bad for health or something. But in theory (hope not scare you) every divice with a li-on battery can explote in any part of the body or poisonus liquid emanates from battery, but it is not too many possibilities that happen xD, those divices are very well tested.
I think I will bought a hush, that is made for the ass (and vagina too).