I don't follow how this implies mandatory attendance makes sense in one case but not the other.

If you believe lack of attendance is "wasting resources", then either you think the class isn't doing its part by teaching what students need, or you believe it is and yet students are not learning the material due to lack of attendance. In the former case, the problem is poor teaching, and so attendance isn't the solution. In the latter case, then the same argument would apply regardless of who's paying.

What's the logic here? Is there a third possibility I'm missing?

The logic is, nobody cares if you waste your own tuition money by not attending class, but people do care if you waste somebody else's tuition money.