Well the obvious differences in their celebrity status aside.
Ashanti was not only performing her lapdance in a more risque stripper-esque costume than Beyonce, she was also performing in a small hole in a wall club type venue that had virtually no distance or barrier between her and her audience.
To top it off she was doing that segment with a backdrop in the form of a jumbo screen LITERALLY playing a video of a stripper on a pole with stacks of money being thrown at her. Plus i'm also pretty sure Bee's act didn't include grabbing the guys d*ck.
Its not an issue of one performance being more wrong morally than the other. I personally have no issue with either, but it doesn't take a rocket scientist to see why Ashanti's audience would feel more comfortable throwing money onstage at her as if she were a stripper, than an audience watching at Beyonce giving a diet lapdance while wearing a ball gown, in arena that holds 20k+ people.