Bitcoin charity fraud - is it illegal?
If you were smart enough to set up the system you describe, you'd have far easier ways to earn $100!

Such activity would show up in statistics as a significant drop in ad clicks or similar measurements. If the operator of the site is canny, they could start to make regular (even automated) changes to the site design to make it tougher to parse. It's a cat and mouse game I guess in a similar way to malware and anti-malware software.

If you dropped the profitability of the site too much, I'm sure it would close.

This question discusses the legality of fraud relating to Bitcoin:
Is performing a 51% attack / double spend on Bitcoin illegal?