I know if I was an SQ member I probably would not use my miles for UA unless I was flying someone (like domestic US) where SQ didn't fly.
Maybe SQ restricts UA in particular because they have so many more members who think SQ is the greatest thing going and want to use their miles that way?
That said, however, on this forum most MP members know using miles on SQ is a great thing. Yet I can't imagine the majority of UA members know or want to travel to award destinations served by SQ. I think I read somewhere that 80+% of UA MP awards are domestic flights.