I really can't buy into this reason because they used to do beta without the subscription requirement. It seems like if they want to test how many people are interested, they can just open it up to everyone like they do for every WoW expansion. And how are they going to know how many servers they need for all the people without a subscription who can't reserve a name or qualify for beta?
This is the lamest excuse to try and pin it on a cash grab. Tbh this is just you complaining to complain, there is no issue. If you wanted to play classic on launch you'd have to sub before hand anyways you're still paying month to month you don't pay by week. The only way you'd lose that money is if you quit before your sub ran out later on down the road.