I don't know why operators are like that
I live in scandinavia and to be honest I have had gigs of data / month, I use my phone a a 3G modem all the time, so do a lot of my friends and noone ever complained. Nokia's PC Suit even has a button to easily connect to the internet via your phone.
There shouldnt be any restrictions period. At least here "all you can eat data" means just that...It's a godsend when my DSL goes down (which happens once a year but still) or when I am at the summer cottage relaxing bu t no internet except my phone.
These tethering restrictions suck!