It's because visitors from those sites are worthless. They may click on the ads, but they aren't going to buy anything and even if they do, the conversion rate is so low that the advertisers get mad at Google.
Google already modifies the payout on clickthrough using a completely opaque system. If that system cannot handle traffic spikes, they shouldn't be defaulting to banning the Adsense account, they should just throttle inventory.
The other solution requires making a lot of changes to a lot of accounts. The advertisers' ad positioning, the rates for the ads, which affect other adsense sites, etc. In that case, Google runs the risk of angering a lot of customers, both advertisers, and real estate providers.
The other solution: ban the account -- angers only one real estate provider.
The most amazing to me is that it's now Thursday, and Google has not responded. Even if they don't like humans, they could have automatically detected by now that the traffic is due to the links.
Don't they have this thing called pagerank that computes exactly that? Another example of the left hand not talking to the right hand (and not caring).
Adwords is not much better by the way, we've simply stopped using it after detecting some major discrepancies (read undetected click fraud) between googles reporting and our own log files.
Complaining is pretty much pointless, we no longer bother, possibly this experience is unique but I highly doubt it.
I guess some computer in googles database records that as 'customer satisfaction going up', but in fact it is the opposite and if I ever come across a real alternative to adwords/adsense we will be gone in a heartbeat.
That Google has no adjustment to apply here other than banning an account -- especially given how many total crap sites thrive on AdSense month after month without triggering countermeasures -- is disappointing.