[omash]

Surely Google detects all the anomalies and refuses to pay? The referrers would probably be suspicious and the fact it's apparently coming from the official google.com site/results page and not from wherever the bot masters are pretending the traffic is coming from.

[tyingq]

Google will often ban the account that's getting false clicks...so you can use it as a weapon as well.

[eli]

Google claims they detect and filter bot traffic, but it's very difficult to determine how good they are at it from the outside.

[jbooth]

A few years ago at least, they were regarded as better-than-most which is definitely not to say perfect.

A lot of mid-sized ad networks and ad buyers actually have a negative incentive when it comes to detecting fraud -- those fraudulent clickthroughs look great on the aggregate metrics, so if you're the only person flagging them, you look like you're getting your customers a lower CTR than the competition, unless your customers are sufficiently educated.