Heineken do indeed have the contract to supply the beers. However, it's the club who choose which of their beers they sell. Now selling only Amstel along with Dark Fruits, as that's what they want.
Heineken do produce some good premium beers, Birra Moretti, Deuchars, Lagunitas IPA, Maltsmiths Kronenbourg, Caledonian Three Hop, Sagres and Murphys. But seems the club don't wish to stock any of those.
I get it with cask ales, as if not sold on the matchday, any left is waste by the time the next game comes around. Besides, I truly doubt that any of the current staff 'looking after' the beer lines and such right now, would remotely be able to keep cask ales anyhow. But bottles and cans of the above are easy to store and serve.