Sometimes...the shoes are just squeaky. And when a person moves, the shoes squeak. Asking the person not to move in a tennis match is...well, kinda unrealistic.
And some courts tend to be 'squeakier' than others as well, for instance, I played on a really slick indoor court this weekend, and I was squeaking like you wouldn't believe (my shoes are also worn down). Absolutely none of it was really under my control and none of it was 'intended'...but I'm not going to *not* prepare for the ball just because I think my shoes might distract my opponents. I believe someone else also said it...if that bothers a person, they have much bigger focus problems that they need to fix.
I think you'd always have a hard time trying to prove 'intent' if you think someone is doing it deliberately while you're serving as well. Some people split-step, and some people move their feet more than that while getting ready to receive the ball. In my opinion, it is just too minor of an area to get too worked up about. *shrug* Your mileage may vary...
What would you do if you knew you could not fail?