Likely some sort of CYA thing. I don't make a habit of removing the chain on a regular basis but I've had no issue at all with undoing it and re-using my Powerlinks a couple of times during the life of a chain when work on the bike calls for taking the chain off.

I've always found that my Powerlinks snapped into place nicely if I pinch the outer plates together firmly while pulling on the chain and slightly flex the links side to side.