Agree that standing is safe (with adequate barriers, no fences, good entrance and exit flows etc) but safe standing is even safer.
Let's be honest, the authorities aren't going to sanction a return to bare concrete terraces so why not have safe standing? Its much much safer than standing up in seated areas, which is downright dangerous as if you're going mental for a goal its east to gash your shins on the seats around you or even over balances and go over the seat in front.
Don't seat why it has to be one thing or another. There are a significant amount of people who want to stand and this safe standing / rail seating gives them this option