If they're holding back game altering content like missions or elites that are sitting in the game files, I would see reason to be annoyed.
Can't see an issue with selling skins though, it's just a little extra that they experimented with at the end of dow2. If they sell them for stupid prices nobody is going to buy them and they'll change their tune.
With the hate this game has received from some people I don't think selling content that was complete before release as DLC after release would go down well, it's probably the reason we haven't heard anything at all about the skins even though they were visible on release.
I am not fussed, its just skins so wont make a difference to me, I was happy to see skins used as a way to keep funding the game after release in the hope of seeing continued support/content but if those skins are going to cost money, it was a really bad idea having them visible to players on Day 1 (plus the accidental slip up on that stream).