[IsoldesKnight]

The answer is probably related to why a paid premium tier negates the value of an ad-supported free tier. A person who can't (or won't) afford the paid tier is probably not going to pay an advertised product, and thus the consumers of the free tier are less interesting (e.g. valuable) to advertisers.

For value to exist in the free tier, it must reach the entire audience. Think about how radio or newspaper advertising worked. If one segment figures out how to avoid the advertisements, it reduces the value of the remaining audience more than proportionally.