A lot of S&M girls (like myself) do massage as well and some S&M girls offer escort services but it seems to me that the vast majority of S&M girls who advertise as such are tops. That being said, it seems like there are a lot of escorts who are into both sides of it but don't really advertise as S&M.
OK, I know what S&M is and have dabbled in it a bit. But, when I see a provider that is labeled as Escort, Massage, S&M I wonder if that means the provider administers the S&M or is a receiver. I lean toward being a Top but have allowed a girlfriend to spank me. Actually didn't even register with me, so that wasn't a turn on for me.
A lot of S&M girls (like myself) do massage as well and some S&M girls offer escort services but it seems to me that the vast majority of S&M girls who advertise as such are tops. That being said, it seems like there are a lot of escorts who are into both sides of it but don't really advertise as S&M.
...or you can just ask the provider. There are ladies who switch full time, ladies who sub full time, and ladies who are predominantly dominant that switch with certain clients.
Spanking isn't the only bdsm activity, either. perhaps you just haven't found a scenario that suits you yet. 
This topic has been discussed before on this Board. I think the majority opinion was that most reviewers don't know if a provider provides S&M, so they just check the box under the assumption that she might... to err on the side of caution.
OR,
Reviewers interpret "She didn't mind that I spanked her butt once during sex" to "provides S&M services".
....almost infinitely more difficult.
It forces an endless series of careful readings of dozens of reviews of the same provider, as well as careful perusal of each provider's website or other promotional vehicles, hoping to see a glimmer of intersection of interests.
And then there's the always fun guessing game of "Gee, her website only indicated in four different places that she doesn't offer sexual services. I wonder if that means she doesn't offer sexual services?" (My experience has been that four or five different statements to that effect usually means about a 50/50 shot as to whether or not the lady in question offers sexual services to at least some portion of her clientele. Personally, I'm not inclined to "take shots" in hopes of getting the experience I seek. This is kink-hobbying we're talking about here, not BDSM match dot com, and I'm not going to pay $XXX just in order to find out if "she really means it"--which is pretty much of a lose/lose situation, and hence very unfortunate for all concerned.
I mean, it's even worse than that in some cases. Just as there is continual haggling about reviews, fake review posting, offers of discounted time for 9+/9+ reviews, etc, there are even "BDSM-no sex providers" who do offer sex, but don't want that part revealed in reviews. It's all pretty f'd up....)
sigh...oh well...someday a better mousetrap will be built...
Or you can just directly ask. Someone who isn't honest and who you have to weasel an answer out of isn't really someone you want to see, is it?
I agree with you completely and have had some of these same experiences. I used to primarily see traditional pro dommes, but many of those ladies are too clinical and won't provide any kind of sensual ending. I really enjoy BDSM play but there should be some reward at the end. Being told to jerk off in a corner after paying $$$ is not how I want to conclude a session.
I've had much better experiences with kinky providers on TER but not all of these ladies have the BDSM skills, gear, and fetish clothes that I'm looking for.
...I got very very very lucky when I first decided to see a pro-domme, because she's one of the few who does provide a "sensual ending".
And you're right about how kink-GFE/TER providers often lack the skills and gear. I'm not criticizing--but can you imagine anyone but a pro-domme owning and knowing how to use catheters and sounds, St. Andrews crosses, e-stim devices, vac-beds, etc? This kind of equipment is expensive enough as is--it's impossible to imagine anyone but a "specialist" investing the time and money into it.
I am sure there is lots you have yet to try...
TER should have an option on the providers profile stating:
Top
Bottom
Switch