both my boys sit with their legs in front and side saddle too! Sebastian, occasionally, will even sit with his frog legs behind him! I'll try to get pictures of him next time he sits like that.
What a great mug!! yeh, maybe it's because they don't have a tail and they can sit straight on their bum. I have an EB/cattle dog mix that does it, my mini does it but my standard does not - he does froggie though - so cute.