First off, if a provider is TER Reviewed, it doesn't really matter to me where she advertises. Since I've found TER, I, personally, won't see a provider without reviews unless she comes from a great agency I frequent, or is recommended by someone I know.
That being said, there is one very good explanation for why TER members may troll CL for providers who are not reviewed. It's by no means the only explanation (among other reasons, there is also saving money and lack of TER presence in some areas).
Prior to TER (and other review sites not as good), the hobby was, again IMHO based on my experience, divided into three general areas of a spectrum. You had Men's Clubs, Brothels, and Call Girls on the high end, the marketing of which was usually done by word of mouth in shared secrets; a benefit being that specific information about specific providers (including prices and services) could be passed along and taken for what it was worth depending on the person sharing it. (A lot like TER now in some ways)
In the middle of the spectrum, you had certain bars, the Yellow Pages for escort and massage services, and some local publications. The marketing for that was larger in scope than word of mouth, so it reached more people, but there was no real sharing of specific information other than price and contact information. However, there wasn't the same level of danger that was associated with the lowest tier, the streets, dive bars, and truck stops.
When you cruised the streets, dives, and truck stops, you could actually see what you would be getting, but had no idea whether she was a cop, ROB, or a stringer setting you up for a mugging. You also had no idea what she would do or not do as far as services went.
In this day of internet and review sites, you have a combination of the best of all general levels of the spectrum. Specific information can be shared, you can see what she looks like, and she can market to an extremely large target audience. Add boards like this, and you can even interact with her before you set the session if she is active on the boards.
What's missing is the danger and, yes, the level of skankiness of the old days that is a draw to some (I'll admit that it includes me on rare occasions, although I no longer act on the draw) The adrenaline rush that used to come with cruising the dives, truck stops, and, more rarely, the streets, is one that cannot be duplicated with any other activity. It's not better or worse, just a different kind of rush. You have to be hyper aware of the surrounding, constantly reading body language and faces, while at the same time setting up a time to be completely alone with someone.
Unfortunately, that level of the hobby also includes a desperation and hopelessness on the part of some of the providers that appeals to certain types of hobbyists that I hope we don't have a lot of on TER (unfortunately, I think we have some). CL, while not as dangerous, has that element of the unknown and access to the skanky, desperate underbelly of the Hobby as it is really just one level up from the streets. Some guys get off on that danger element and that rush of trying the unknown in hope of striking gold. More power to them, I can't afford the trouble, but that is one reason guys on TER will continue to go to CL again and again.
Good luck