They technically have the right to, but i know my fair share of mobile devs and i've never heard of it actually being enforced.Apple Does Not Keep the 30% Commission On a Refund [Update] - Slashdot
From reading the Apple developer forums and looking at the agreements...Apple has the right to keep its 30%, but can expect the developer to refund the full cost. So, if I purchase an app for $1 the developer gets $.70 and Apple gets $.30. If I request and am granted a refund Apple can charge...news.slashdot.org
The right being there is an issue, and it speaks of the usual pattern of vessatory conditions with inconsistent enforcement, but it's not common policy
When I've spot checked this personally in the past, they've been exercising their right with us. Admittedly, probably 5-6 years since I last audited it as I've just taken it as a given.
I have a new task for the week I guess.