I have the same issue with spotty 3 G coverage and would rather just switch it off when I'm around this area to stop it trying to connect to the network whenever it detects a signal.
I've read up on this before and seen people claim it's carrier specific, never once seen anyone actually provide proof though. So until they do I'll continue blaming Apple for being pig headed and trying to push 4G on the newer handsets.
You are correct, it is not carrier specific, and is completely Apples' fault.
It's the way IOS is designed, you can have only 1 switch, used to be either 3g off or 4 g off, now in IOS 8 it appears Apple made 4G off default state so even if your carrier does not support 4g at all and never released any settings you will still have 4 off instead of 3g off in you network menu.