It's a kind of 'necessity' filtering, I think. It also probably keeps down their call volume by preventing people from calling in with stuff easily solved using other channels.

I'm not saying whether or not it's good practice, but if it leads to the quality of these phone support calls, then I'm all for it.

If anything this post and it's related comments seem to prove email ISN'T working as a support channel, phone is fine though, but it's relatively hidden.