I am a mom, and managed to keep my kid from screaming while in public places OR LEAVING IF SHE COULDN'T. There's no reason that parents need to "ignore" behavior like this hoping it'll go away, and I see nothing wrong with someone putting up a sign like this. Well actually, I do see something wrong with it. It shouldn't even be necessary to put up the sign; it should be common sense to remove a screaming child from a public place where other people are trying to live their lives.
And in regards to the autistic children; I can totally relate, however autistic children are certainly NOT constant screamers (I should know, I worked for several years among 20-30 autistic preschoolers!) and they can be worked with and controlled or taken out of the situation, if their parents care enough to.
IMHO, of course. YMMV.