That's because in Europe and many other parts of the world servers are paid a fair living wage. I don't know what the hourly wage is for servers at WDW (if there is a CM who could verify here, that would be great), but the minimum wage for servers in Florida is 3.77/hour. Minimum wage for any other job in Florida is 6.79/hour (and it's almost impossible to survive on that minimum wage). So they
really do rely on tips as their income.
Although it may come across as rude or pushy...I kind of understand why a server might nervously remind guests that the DDP no longer includes gratuity, or ask a guest how they wish to pay for their tip (especially if the guest doesn't typically carry cash, and may have assumed the tip was still included in the plan). It's awkward, clumsy, and uncomfortable all around, and I really don't know how to make it better.
A particular percentage for a tip should never be assumed by a server, and a tip can certainly fluctuate based on how good the service was, but it's understood in America that diners pay a large part of the server's wages with the tip.
I plan to follow the advice others have suggested on this thread and if I am asked I will just tell the server that I am paying cash for the tip, and will figure out the exact amount when I am ready to leave. I will say that pleasantly, with a smile, and hopefully the CM will get the hint that if they do their job they aren't going to be slighted