There is no difference, they're just making up far fetched excuses to gouge more money out of customers.
AFAIK it's only in the US that carriers are doing this. I was told it's because American carriers equate "unlimited" data plans to all-you-can-eat buffets, that "unlimited" is just a marketing term that shouldn't be taken literally(!). Sounds like a scam operation if you ask me.
No it isn't just the US carriers, O2 are just as bad in the UK.