Well, the law could require allowing anyone with a different view point to be able to advertise (such as when there are more than two viewpoints) . The "sponsored by Budweiser" ad you describe is exactly the point of the law. If everyone gets equal time broadcasting their views, then the influence of being able to broadcast your view is drowned out. It would certainly allow all kinds of crazy response ads. The point though would be that fairly quickly media would simply stop accepting all paid political ads. It wouldn't be in their financial interest to continue doing so.
The "loss of the airwaves" and cost to the media would just be too bad. They could choose to not show any political ads, and therefore not have to worry about losing control of their ad time.
Regarding the limiting of free speech, I can see that argument being made, but counter arguments can also be made. There used to be a fairness doctorine regarding radio (abolished during the Regan administration). In the past the Supreme court ruled that it was acceptable for radio. I believe such doctorines were shot down for newspapers on the principle that it violated freedom of the press in some way and that anyone could in theory create their own newspaper (which is not possible with radio stations). (I'll admit my limited knowledge here is from wikipedia and may be incorrect.)