Cabrio Rollbar Question

Now that I've had my VW a few months, 1995 Cabrio, I can affirm I'm
still in love with it, and it's the best suited to me car I've ever
owned--new or used. Anyhow, now that the weather's dictated I travel
more with the top up than down, something got me curious:

The top of the rollbar has a thin layer of slick foam rubber stuck to
it. I can be entirely sure, but it seems as though if I were to peel
it away, there'd be nothing but black plastic underneath. The reason
I'm wondering what the hell it's actually there for is because it
"shimmys" against the interior headliner of the roof when it's up and
end up making a squeaky Styrofoam-coming-out-of-a-box sound...
PARTICUALRLY in "dry cool atmospheric conditions."

What purpose does it serve?

I'm not sure if you can tell very well from this picture, but here it
is.

http://www.intrex.net/jm/foamontop.jpg

--S. John