Ahh, but you're mistaken. Curse hasn't developed the majority of these addons. Lots of third party developers have, and they had chosen to host them with WowAce (mostly because they used the Ace library as a foundation for their mods). The people who ran WowAce (who work for Curse now) decided on this move.
Which was their choice as they run the site, if the addon developers don't want their addons to be apart of this they are free to remove them.