When I was in Aruba this Spring I took a Gypsy Bus to go downtown. Mostly locals on it but along the way we picked some other tourists, one of whom was rather loud and possibly drunk (it is afterall "one happy island") and this woman started asking loudly if we should tip the bus driver. It was all I could do to say there was no need to tip someone operating an illegal service. I think the locals were upset too. Now I don't know for certain that this is an illegal service, but it isn't sancioned by the authority that runs the public transportation service in Aruba.