What's wrong with that? The admins are using their servers, their software, their database, their bandwidth, etc to host the forum for free. They have to get their profit from somewhere and if a video click provides it, then why the hell not? Chances are the ad was only for a couple of seconds anyway so what is the problem? They still provide exceptional service for a free forum host.
As for the video having ads, did you forget that Youtube does it, Penny Arcade does it, The Escapist does it and practically every-video-site-ever does it. Hosting websites isn't cheap and they need to make money somehow. Getting a cent from every sign up is just a simple way to do it. You're not even the one giving them the money, it is the advertiser that is.
I find it unlikely that your members have not come across a spambot before and even so, if you promptly remove the spam and explain the situation then I doubt they're going to care very much. That way you still care for your members while not ostracising potential ones.