Well, strictly speaking, this is a legitimate dictionary definition that doesn't involve money:
> a person of a specified kind with whom one has to deal
[edit] Youtube has 3 kinds of customers (or users, if you wish): publishers, viewers and advertisers. It makes sense to separate them like that, but not based on a "pays/ does not pay" criterion. If PewDiePie is a customer, then most definitely Blender Foundation is a customer too.
> a person of a specified kind with whom one has to deal
[edit] Youtube has 3 kinds of customers (or users, if you wish): publishers, viewers and advertisers. It makes sense to separate them like that, but not based on a "pays/ does not pay" criterion. If PewDiePie is a customer, then most definitely Blender Foundation is a customer too.