Our factory cruise control works the same way as you describe.  It's just something you learn to live with the way it is.  Ours also "kicks out" when you go over a big bump in the road.  I can usually feel the cruise control take over while I'm holding the button in - the acceleratgor kind of goes down a little while your foot is on it trying to activate the cruise control.  Another oddity about our vans!

Sharon Mendonca
88 Vanagon GL
88 Westy