I think it might depend on the offer as well. I'm probably the same on most things. But AppSumo has a similar "Free Deal" on their site where you get access to the video/product after tweeting about it. Since I may actually be interested in the offer most times, I'm usually pretty happy to do it. I know that I'm going to get value out of it and, by helping advertise the site, I'm helping them continue these offers in the future.
Then again, I'm probably not the target for a viral marketer, so perhaps this is an effective solution.