If its for our own safety then surely we shouldn't be able to stand up at half time either.... that's just as bad isn't it??
Does something exciting happen at half time? Are there things that make a full stadium jump up and down and take leave of their senses momentarily? Are probably more than half the fans out getting burgers or using the loo during the match?
During a game there are controversial incidents, dubious ref decisions, hearty challenges, there are goals - the sorts of things that excite a crowd and make them forget their surroundings and the safety rules.
How often do you, while celebrating a goal, jump up and down then stop and think "hang on, should be careful, if I lean forward to celebrate with my fellow fans in the row in front of me, or the next row down I could lose my balance and land on them, squashing the 86 year old man there, or the 8 year old girl over there"? You don't do you? You're lost in the euphoria.
Extreme examples, I know, but doesn't mean it won't happen. Maybe the FA are being too safety conscious like most of modern society, but personally, I would prefer that to another hillsborough.