In any case it doesn't really matter whether Amazon has the biggest share of the market or simply a very big one. Because adding epub capability won't cause them to sell more books to Kindle owners. It might cause them to sell more Kindles - but that's only a good thing if Kindles have a decent profit margin on them - and we already know they are priced virtually at cost in order to promote sales of books.
And in theory there's nothing to stop them selling epubs alongside Mobi/KF8s for other devices. So if they wanted part of the epub market they could go for it. (I suppose it may be this is seen as anti-competitive but I personally can't see it's any more anti-competitive than having a walled garden in the first place, but I don't know the law well enough to say for sure.)