The guides are megaposting on TA for the primary purpose of promoting their business (under the guise of just trying to be helpful to tourists). They don't explicitly self-promote so as to stay within TA's rules......,
I think that you are exaggerating this conflict of interest. If you check the comments posted on Tripadvisor by these guides, they never suggest to anyone who wants to tour on their own that they must hire a guide. On the other hand, these guides visit the tourist sites daily and are able to provide much more accurate and up-to-date information and general advice than persons such as myself (I am not in the travel industry).