Well, there must be a limit to the crosswind capacity of each aircraft. We are just arguing about how that limit is determined and specified.
If I were trying to sell an aircraft which I had manufactured, I would be keen to specify as high a figure as I could. So, I would guess that when Piper quote a demonstrated 17kt, or whatever, they are stating the highest figure which they reasonably can. I would also guess that they have half an eye on the law and possible actions which may be taken against them. The court would presumably look at what it is reasonable for the manufacturer to claim. Doesn't help much, does it?
I treat the Demonstrated figure as a limit...you've gotta have some tangible figure to work with.
I haven't seen a POH that specifies a speed increase when encountering a crosswind; .....I don't say that one doesn't exist, mind you.