I've tried defending them before, but this is dirty pool.
A downstream ISP that contracts from Bell pays for 3 things. An IP Block (the addresses clients use to connect to the Internet), a connection and (probably) bandwidth rates.
All three of those cover Bell's concerns about the usage on that pipe they just sold.
If the downstream ISP wants to re-sell "unlimited" access to its clients, it is free to do so. If all of its clients total bandwidth exceed the monthly cap under their agreement with Bell, they pay overage charges. There's no way Bell can decide what the usage is of customers of the smaller ISP.
Unless that is what this all about. Bell is looking at adding caps and overage charges to the ISPs buying access to Bell's network. THAT is legitimate.
But they cannot decide what the bandwidth cap is for the customer of the independent ISP. That's overreaching their authority.
PS This is different than rate limiting traffic on the backbone, like P2P traffic. As explained in my previous defenses, that is a different issue that could be legitimate.