There is soap in foam... you can't just spray water with a foam cannon and get foam. Why is it I can foam the car, take my fingers, scoop off foam, and it's slick, oh yea BECAUSE THERE'S SOAP. It's a lubricant.Foam isn't soap. It's air.
Water is a poor lubricant. Unless you're talking about caked on mud there's no need to pre rinse a car any more with modern soaps.
If you are talking about mud, the damage is already done.