The DME motorcoaches are not equipped to handle standees, and thus, they are strictly prohibited.
Most likely the driver just thought the more people on board, the more tips he/she could get. (Uh, yeah, NO.)
In this case, Edd has a good point. Anytime something seems weird, it's a good idea to write down the information just in case it really shouldn't have happened. If you do happen to have the bus and/or driver information (or even just your pickup time and location, they can look the rest up), it would be a good idea to contact Mears to report the situation. Mears cannot fix a problem that they do not know about.
At the very least, they can post signs around thier base saying that standing on the DME coaches is prohibited and will result in disciplinary actions... and at best, they can find the driver who allowed it and take care of the situation.