> In reality, they should "award" grad students $0 in tuition and pay them like what they are - employees (and deeply exploited employees at that!).

It's not always the university paying the tution fee. If the PhD student is a teaching assistant, it might be the department. If the student is research assistant, it might be the NIH or whoever is funding the project. This could be public or private (e.g., Google Faculty research awards).

I seriously doubt that in any of these cases, university would be able to claim $50k as gift since the university actually received tuition fee from the student although paid by someone else.