Yes, we can say, "I consent to viewing an ad in order to receive X free service" in the same way that we consent to viewing a commercial when we watch TV or listening to an ad on the radio.
However, in those latter two examples, the information is one-way. Those advertisers don't collect any personal information (outside of perhaps our viewing/listening location).
When it comes to website ads, most consumers do not know/realize that a) the advertisers are collecting a WEALTH of your personal information and b) that information comes at a cost of your bandwidth (which, for many mobile users, is limited). There are probably many other things that happen between the end-user and the third-party that I am not aware of.
Sure, they may consent to viewing a free ad, but most of them do NOT consent to collection of information nor increased usage of bandwidth.