This is most likely only due to your ISP:
E.g. when I'm in the internet using a T-Online dial-up, I can read the
news on the newsserver in the Technical University of Berlin (TUB),
but I can't write anything there. But I do have full access to the
newsserver at T-Online.
When I'm using a dial-up via the TUB, I do have full access to the
TUB's newsserver, but only read access to the T-Online-newsserver...
I guess that your problem is similar - most likely your more expensive
ISP has signed a cooperation contract with the newsserver-providers
and hence doesn't get blocked.
Or your cheaper ISP is on a "black-list" due to some spammers :-(