they arent asking. they are saying that you are helping them by clicking on a banner or signing up with one of their services etc. atleast thats what i think if u read just "click on a banner or signup" then it seems like they are telling you to click a banner but when u read the complete thing-"You are helping....click on a banner or signup" and then u will get what they actually mean
I personally feel that it is against google tos to say "click on a banner". This is one of the marketing gimmick to make the people click on the ads and sometimes it might discourage the people not to click on the ads.
People click on ads because they are interested in the product, not to support some shady sites. This is definitely against the tos.
But indirectly they are asking people to click on the ads. It's like saying "support us by clicking on the ads and we will remain free".
SiteMeter is a good company with a good product. Someone should just tell them to change it to help them out before Google freaks on them.