The glib answer to this, of course, would be, "Yo, you're buying a hotdog. That thing is 90-percent ass already, there's no making it grosser." (A more serious response would be, "Yeah, and what if you touch your FACE before selling a hotdog, your face looks like a butt.") Because barring any, er, penetrative butt touching, the face is very much the more risky source of germs -- which is why all food vendors are supposed to wash their hands, and why the hotdog ladies' carts included the same mandatory hand-washing stations as everyone else's.
A second objection came from those who figured the sexy getup was merely an advertisement, with actual sex the product for sale. Clearly, these women were having sex with customers, taking in just as many wieners as they dished out.
Police did sometimes accuse them of this, as a part of longer lists of vague offenses. But when newspapers inquired after more details, police admitted that they'd never arrested any hotdog vendor for prostitution nor appeared to have any evidence of it occurring. A fair number of the women had experience as strippers but not more explicit sex work.
There was a third objection, though, and it was that the women might be killing people.