Stephen R. Walli writes about IP, patents, open source, and litigation in a piece titled, "When are you going to sue your customers?"
Customers have even been known to sue their vendors in specific situations when the vendor fails to deliver on the promise in a contract. Oddly enough vendors never sue customers in any sort of broadly applicable way. There is a really simple rationale behind this. Once a vendor sues a customer, they have essentially told that customer they never want that customer's business again. That might even be appropriate in a narrow situation where there exists some sort of explicit dispute between exactly the two parties. If however the dispute is over something like “patent infringement” that can easily be applied broadly to many customers, then all the vendor's other customers are put on notice that this vendor does not care about the relationship and continued business.
Mr. Walli even brings up the two suits SCO filed against its customers, Daimler-Chrysler and Autozone. I never really understood why they sued these two customers. This action makes more sense when your business model is not really selling products and services, but a "legal play to siphon money out of the system."