Signups only paid if surfer stays 4+ hours

[Ms Naughty]

I signed up to a sponsor this morning, but now I've looked more closely at their terms and conditions, I'm reconsidering whether I should promote them.

Their terms said:

"[Sponsor name] will pay the Webmaster $35 per sign up referred, as soon as this is not cancelled within the following 4 hours after the registration, and an access is confirmed, within the same period, to the selected content."

Now, I can understand why they would do this, as they don't want to pay me $35 when the surfer has just paid $2 and bogged off again...

BUT I have to say, after going to all the effort of pre-selling the surfer and getting them to to the tour and eager to sign up, it's not my problem if they don't stay for more than four hours. I did my job...

Do other companies use this "minimum time before cancellation" thing?

Do you think it's fair?