You are absolutely right. But where nobody else is getting paid then it's a different matter.
On the other hand, I wouldn't personally do panels at a convention where I wasn't at least getting two weekend memberships out of it. We all have our limits when it comes to what we will do without any recompense at all.
Whereas I'm happy to pay the membership on the spec of a panel place I might not get (to be fair, I have a lot planned for the weekend, including meeting some of the military sf guys who've been supportive - and I'm listed as a British author of note in a new book for military sf, so that was lovely news over the weekend - and some Chronners, so I'm going for more than just exposure.)
Your top or bottom line is affected by where you're at, career wise, I think.