Frames can be placed no matter what's on the surface of that block.
I didn't find a matching issue report so I made this one. Tell me if there already is one.
MC-1066
Frames don't care about what is attached to a block. They get placed no matter what is there.
Resolved
Works As Intended
0
Meta
(Unassigned)
Unconfirmed
Minecraft 1.4.2
Retrieved
Item frames and trapdoors are not whole blocks. And item frames are entities. I think its a good feature.