Well, you are all familiar, I am sure, with the Rainbow stickers, flags, etc that are used to represent Gay Pride.
Amongst those that are into Domination/Submission, and to a lesser extent, Sado-Masochism, the stickers you described have the same meaning.
Out here, during the Folsom Street Fair last year a friend of mine bought several of the stickers, and was giving them out to other friends that are interested in the same thing.