I suppose they could do this even with ports - they could have small adds pop up in the corners, overlaying whatever is playing. They could charge $5 a month to disable the ads.
But you're right this is quite possible, perhaps you will be able to buy an ad-free model at a premium, like with Kindles.
Probably some ads would be preloaded, so you'll end up giving it wifi just to get new ads instead of enduring the old ones over and over and over again.
It would be hard to achieve unless it was somehow necessary to connect the TV to wifi in order to use it at all (even when something is plugged into a port). That would be pretty confusing for people I think.
