Cactus can be placed under water, but doesn't break

Now, I know that cactus can't normally be placed like so:
[a][b][a]
[b][c][b]
[a][b][a]

A=Air B=Block(Any) C=Cactus

But for some reason, you can place cactus on sand underwater. It also doesn't break!
Is this a bug?