Arena : No more Yahtzee, but only YAMS rules ??

[ufm]

Problem is I paid for premium for Arena access to play certain games.

Playing Arena mode does not require premium membership unless the game is already a premium game. Yahtzee is not.

And it shouldn't be labelled Yahtzee when it is yams because yams is not Yahtzee.

You should have checked the table options first...
After all, Yahtzee itself is just a commercial variant of Yatzy, Yacht, or other similar dice games.
And they are similar enough to be grouped in a single implementation.