Many career paths include a steady increase in future earning potential. As for entitlement, they are entitled to a share of the value they bring to their employer. Or at least, if Stanford doesn’t think so, it can fire them.
I disagree. You are entitled to what you can negotiate as your market value and what you accept in exchange for your labor and talents. If grad students are lined up the door willing to work for less because a PhD from Stanford gives them a ticket for the rest of their life, why should Stanford or any employer have to give them a share of value? That's not how most employment works. Or at least not O(n) proportional to the value they create.
Organizing and demanding pay collectively seems to help in negotiations, so good for them for taking this approach. It's not like Stanford can move its grad programs to Mexico like a US car maker and its factories.