Usage-based billing (UBB) is extremely common.
Ever have a cellphone bill you by the minute? UBB.
Ever have a cellphone data plan that had a limit on your monthly usage? UBB.
The problem isn't in offering a UBB plan. The problem is in the CRTC forcing that to be the only choice.
Can you imagine the CRTC saying telcos are not allowed to offer "free" (all-included) long distance calls? That they're not allowed to offer unlimited texting bundles?
Do you understand the problem now?