Personally, I don't like nudity or skimpiness (as you can probably tell from my site!

Yet no-one ever bothers to label anything "clothed", "skimpy", or whatever.
As to the topic, I think the reason this "must label male content" thing comes up so often on WAM forums is because WAM is unusual in having a very high ratio of watchers to do-ers. My impression of various other fetish scenes I've encountered has been that most of the people in them do at least a small amount of the relevant activities themselves. In WAM on the other hand, there seems to be a vast number of "watchers" who either don't Splosh themselves at all, or only ever do so solo (i.e. no partners present, never mind anyone else) and in strict privacy. That's not meant in a bad way - TBH WAM lends itself to solo appreciation rather more than many fetishes do - but it may affect people's perceptions.
Given that the UMD is the main commercial portal (and TBH we're lucky it's there, not many fetishes have such an effective and well focussed portal, as Noise said), it's bound to have an even higher ratio of watchers to do-ers, and people who are looking for commercial products possibly aren't entirely happy finding "amateur" stuff, especially if it's "not their orientation", popping up in the middle of their searches? Just a thought.
Lastly, it's foolish to try and lump all Americans together - such a vast country has enormous differences of thought, belief, and practice from one area to another. I've been to a US city where the general attitudes and values of the people I met were more European than some Europeans are!