No collision check for mob spawns.

Are mobs spawning in naturally this way?

Yes, this was tested for structures only.
In the structures they also spawned on the glass or open fence gates.

Confirmed in Windows 10 1.2.10.2. However, I can confirm it only for wither skeletons spawning in the Nether.

I also tested in the Overworld. With slabs (either bottom or top) above a 1-block gap, only spiders (and bats with top slabs) spawned. With slabs above a 2-block gap, endermen did not spawn.

endermen ignore water on top of a slab

I did some testing and noticed:

• Glass (at head level) doesn't prevent mobs from spawning, but solid blocks do

• Enderman can spawn at 1 or 2 blocks gap