I'm surprised to learn this is such a common thing that it has a name "California No", and it also reminds me that there is something wrong with the people of California :)
In my experience, someone who can't say no is not being polite; they are being quite rude. Stringing people along is rude. It is far more polite to outright tell someone that you are not interested and then everyone can move along. Even for dating, I can't speak for other people, but a simple "not interested" is more polite than being ambiguous. There is no "growing number" of folks who react violently because violent reactions, in general, have decreased over time -- and if anything, a clear "no" is likely better than being unclear. In my experience someone would be more angry at being misled over a period of time.
In my experience, someone who can't say no is not being polite; they are being quite rude. Stringing people along is rude. It is far more polite to outright tell someone that you are not interested and then everyone can move along. Even for dating, I can't speak for other people, but a simple "not interested" is more polite than being ambiguous. There is no "growing number" of folks who react violently because violent reactions, in general, have decreased over time -- and if anything, a clear "no" is likely better than being unclear. In my experience someone would be more angry at being misled over a period of time.