As localdriver has already stated, all the buses have these signs, so obviously not against any rules.
But......
Here's where I will disagree. I see the signage as I board the bus, I see the signage as I sit in my seat, looking forward. I see it when I disembark from the bus....I get it, I truly do. But, once that driver opens his/her mouth and tells me that gratuities are gratefully accepted, I put my money back in my pocket. I will not be guilted into offering a gratuity. I have had my carryon bag stowed underneath the bus and tipped, I have had my bag stowed underneath and not tipped! I have had no bags whatsoever and still tipped! I had the most wonderful driver..he had us all laughing and thoroughly enjoying the ride to WDW...he got a nice $5 from me, just for the wonderful experience....no bags, he didn't 'ask' for a tip.
I don't automatically tip housekeeping...if it is warrented by really good service, then I leave a little something. Same with bus drivers....they are paid to drive me from MCO to my resort. If they do something to make that experience better, then they will get something from me. Same thing at a restaurant. I realize the servers are paid very little to begin with. But, if they want that 15-20% tip, then they had best do something to earn it...and I don't mean just bring me my food. They need to go the extra mile. I don't mean that in order to get a gratuity from me, someone has to stand on their head and spit nickels, but they do have to be personable, give good service and make whatever experience I'm having, a better one because of their service.
So...long story short..do I tip the DME driver? Sometimes, but it not a given for me.