Normally once you set a modem to bridge mode you have to use something like a console port or USB port to access it.
Most DSL uses PPPoE to authenticate users, this means that your connection is point to point and you should not be able to see other uses. On top of that almost all DSL runs ATM behind the scenes which means they are running virtual circuits which also keeps user traffic apart.
What they may be doing is running DSL on the edge and terminating it to a ethernet switch instead of ATM. If they also do not run PPP then you get everyone on the same broadcast domain.
This is very bad they do this. You may even be able to monitor other people sessions by doing ARP poison man in the middle attacks.
I suspect the ISP has a bunch of dumb asses working there so make sure you have your firewall settings on and disable any and all file sharing.