If you actually get a ticket to the convention and don't attend, I see no problem.

If you don't, then I believe it's unethical. It's like claiming to be an AAA member when you aren't. The convention organizers negotiated a rate for their members, not for you.

With that said, there is no harm asking if they will honor the rate even though you aren't attending. If they have rooms to fill (unlikely if the convention is big), they might let you.