You're loading the destination sites for the user - you're providing them with the data. It's up to you what you do or do not provide, and what you provide differently for the sake of the service - such as changing the advertisements. xD At least, that's how I see it.
I've been trying to make one, but its tough because there are so many variations to what a publisher could put down. But then again, I'm also pretty bad at RegEx