I hear it, or the substitute 'th' here in Maryland, and on professionally recorded music. I believe that singers and other people who are recorded are taught that 's' comes across the microphone as a very sharp hiss, so they soften it to 'sh' or 'th'.
Which brings me to something that has driven me bonkers for a long, long time: singers using deliberate mispronunciations and fake accents. Why? I don't get it and it's annoying and distracting. Two really big offenders:
- the word "baby" being pronounced bay-bay, and
- the word "want" being pronounced like "won't," apparently in an attempt to mimic a southern US accent. The Rolling Stones sing You Can't Always Get What You Won't, for example.
It's just so fake and affected.
ETA: Awwww, nuts. I though this was the Little Things that Drive You Up the Wall thread. My complaint isn't really grammar-related. (Still drives me batty, though.)