All tyres have 'tread' - tread is the grippy rubber moulded on the outside of the tyre, over the radial/crossply carcasse.
'Tread pattern' is what we're talking about, not 'tread' - even slicks have tread, just it's a smooth tread.
But Fossy's right - slick, non-tread-patterned tyres are fine on bikes in the wet, put more rubber on the road.
A patterned tread does not assist in grip on wet tarmac, has less rubber on the road and also introduces 'squirm', where the tread pattern moves around and gives even less grip.
Slicks don't work on a F1 racing car in the rain, they aquaplane or skate along on the water which pools-up under them.
But that's because a F1 rear tyre is 355mm wide and flat in profile, so a lot of water can pool under it - a bike tyre is say 23mm wide on a roadbike, or 1.5in on a typical MTB slick and is a curved profile : the rubber touching the road is about the width of your fingertip or less, so you don't get any water build-up underneath it or aquaplaning.
An offroad MTB knobbly does have a very agressive tread pattern, but that's for grip in soft surfaces like mud or sand, not for wet tarmac...
