I think one should avoid even the appearance of impropriety, and that vendors who are not fully transparent with pricing are essentially casting doubt on themselves, and leaving it to our nature to assume impropriety.
With all due respect, I wish the rules and/or objectives that allow it were different. These rules or provisions simply do not appear to reflect what I have perceived or assumed.
In trying to play the Devils advocate, the site does bring together the profit minded businessman with the interested consumer, and, it would be counter productive to inhibit interactions by opposing or prohibiting a profit maximizing tactic of those businessmen. Those businessmen make the aspects of the site, those social and educational aspects that I personally value the most, possible.
Regardless, I really like the idea of posting and comparing prices of these vendors, in spending dollars elsewhere, in an effort to discourage and perhaps stop the practice. In my ignorance, the more it's openly opposed and discussed, the more likely it is to stop. I've seen the frustration of some vendors with all the objections, and I think that's a good thing. For every word of opposition written, there's many more that exist but remain unwritten, and any customer minded businesses know this.