Yes I do think it is rude, yes I do think it is unsafe. I also hate people who hold up their huge iPads or phones to record everything.
on a side note, before we had kids we were waiting for the parade over in a spot in Frontierland when two people with a kid actually PUSHED me over. When I said, "excuse me, I'm standing here" she said "you don't even have children, why do you need to see the parade?" I was so shocked, not only did she not realise (nor care) that I had been trying to have children for over 2 years thru IVF, but adults also enjoy WDW and parades.
Just because you have a kid who wants to see a parade or anything else in WDW doesn't give you the right to take that away from anyone else.
I have two children now and could NEVER push in or put my kids up on our shoulders to stop others from viewing anything at WDW - it's just RUDE!