That is indeed an interesting point, especially for users who have metered Internet service.
Consider a user with a data cap of 1000GB, is metered as consuming say 1200GB of data, of which 200GB is Ads. The user is forced to pay extra for the 200GB to the ISP which the user should not have had to pay if there were no Ads.
Ignoring that ISPs should not meter the service, the above just doesn't seem right. The individual Internet services may be subsidized by Ads, but the user is paying to the ISP. So, in effect a user is paying, just to a different party.
Some lawyer should definitely look into it and see if there is potential for a class-action against Ad vendors.
Edit: I'm sure the flip argument would be that the user requested the Internet services that came with the Ad, so in effect the user is responsible for consuming the data. But that is where I think it becomes interesting and given that IMNAL I don't know what the legal minds think.