After getting sued a good course of action would be to sue the manufacturer for loses incurred. At least that would get their attention. Somebody could start a class action lawsuit agains the manufacturer.
They should be suing the manufacturers in the first place. Even if we were to assume for a moment that this patent has any validity whatsoever, it would not be the users of the wireless router than owe them a royalty payment, but the router manufacturers. And I seriously doubt that a fair royalty on a single router (such as what you might find in one of these small coffeeshops that they are suing) is in the thousands of dollars.
I know, you know, the manufactures know, the lawyers know, but unfortunately the lawyers do not care. They just want to make easy money. This might be a good way to shift the pain to the manufactures.
Don't consumers have some protection here? I mean, they probably bought the wifi at a common store. You don't normally walk out of Walmart with a box containing a product you expect to not legally be able to use.
Legally, it depends. A bit of web searching did turn up (so far) one LinkSys router that explicitly disclaims implied warranty of non-infrigement. If that is disclaimed, then yes, it falls to the consumer to deal with it.
