The way the dialogue in this episode makes it seem, they think being submissive is allowing the client to injure the Active and that's why it's banned.
Being a submissive, in the BDSM sense, requires putting total trust in the dominant. The exact dynamics vary from person to person and relationship to relationship, but they're all about loss of control, about giving control to the dominant partner, and there is a great deal of space for abuse there. The Dollhouse trusts it's clients, but not that far.
In 99% of BDSM relationships, the submissive is really the one in total control of the situation. The lack of control is an illusion, a sexy illusion. At any moment, if the dominant goes too far, the submissive can break that illusion and end the scenario, usually by use of a safeword or gesture (if gags are involved). Usually, dominant and submissive will plan the scenario together, in explicit detail, so that the limits and boundaries are clearly defined. Crossing those boundries can happen by accident, in the heat of the moment, which is why there is the safeword. Ignoring the safeword is the single most assholish thing any dominant can do.
The whole thing centers around the submissive's fantasies and the submissive's pleasure. The dominant's desires are secondary. It has to be this way, because it is so very easy to go too far if it isn't.
A person going to the Dollhouse isn't negotiating boundaries with the active , he isn't trying to give the active pleasure, he isn't trying to fulfill the active's fantasies, he's trying to fulfill his own. And that can be a problem. Being a dominant requires a great deal of responsibility and care.
An Active imprinted to be the perfect submissive would never use the safeword. There would be no such thing as too far. Accidental physical injury is possible. Intentional physical injury is possible. And so is emotional and psychological injury.
It's not exactly hard to hire a professional dominant if you live in a major city, and given they live in Los Angeles that's really not going to be a problem.
On the other hand, true submissives (not just escorts who'll don't mind pink fluffy handcuffs) are not easy to hire.
There are lines that a professional dominant will not cross. If you want to go beyond what is allowed by the standard SSC model and into the far extremes of what is permissible under RACK, including edgeplay, play without safewords, and some other more dangerous activities, it becomes very difficult to find someone who will take you there.
Let's face it, finding a woman who A) will nail your penis to a 2x4 and B) is sufficiently skilled in the art to do so without causing permanent scaring or loss of function is not an easy thing to do.