Most American visitors without transponders will be billed (or rather the registered owner of the car will be billed, via mail) for their use of the electronic toll roads. There is a reciprocity agreement with a number of states (at least a dozen, including New York) allowing access to the license plate databases.
Ontarians who have outstanding toll fees will have their license renewal withheld until they fork over the money in full (including the fairly arbitrary video toll fees and late fees). The fees can mount into thousands of dollars if ignored (source: two acquaintances who disputed fees).
As far as collection from out-of-province deadbeats, not sure what their reach is. There is a reciprocal agreement between NY and ON on demerit points, for example. The bill will be in Canadian dollars, which are pretty cheap at the moment, and it's easy to pay with a credit card.
The normal fees are complex (km and time-of-day based), and fairly high at rush hour, but I agree the video toll fee should be listed on signage- or perhaps an 'up to' amount if they don't want to keep changing the signs.
It's pretty obvious you are entering a toll road, there is signage and a huge gantry over the road with cameras, lights and antennas. It's much less obvious when driving in, say, Dubai.
I would say to pay the bill if you used the road. I certainly have paid for any such services I have (even inadvertently) used in other states and provinces.