What you mean with captcha? You want to say bidvertiser ask ppl to click captcha wehn they click on the ads?
I don't click on my own Bidvertiser ads so I wouldn't know but it's weird if they do ask something like that. It will just annoy people!
Captcha on the click-through is one (very old) way of authenticating the user as real or as a bot. We used to use that method years ago and discontinued it for a couple of key reasons: bots became smart enough to read through captcha, and it annoyed the heck out of users. We found that visitors didn't want to interact with something on a click-through as it lowered their confidence in what they clicked on - they felt they were getting scammed even though it was just random text. Surprisingly, it also significantly reduced conversions. I'm rather surprised they went with this old-school method.
To me it looks as if they are unsure what they are doing. I for one wouldn't bother writing in Captcha everytime I click on a link on someones site. It's bad enough doing it when signing up to sites and all that.