It's because it is only legal to sell and provide the service and it's illegal for clients to seek it out. This gives the provider all the power and is the main reason this can happen and become.commonplace with little to no repercussions. If it was legalized outright for everyone, this type of behaviour wouldn't happen anymore at least not without legitimate repercussions.
Clients would be far more likely to report this type of behavior consistently if they wouldn't get in shit for it. Even though, yes we can report anonymously, if LE really wanted to track us down via IP address, etc., they could. How seriously do the anonymous reports even get taken unless there is an influx of them about the same ad/provider?
Furthermore, why should LE care? As far as they're concerned, the client who got robbed is the one engaging in and seeking out the illegal activity and it is just a price you pay when doing so. But these providers can cry any form of report they want and LE will be on their side no matter what. Be it SA, assault, robbery, harrassment, etc etc. The client is far less likely to be believed. Plus, the client getting involved.in a situation like that could result in unwanted publicity that undermines the discretion the vast majority need to partake in this.
OP, I'm sorry for your loss. I've had a couple situations like that happen too, mostly in my first few years before I became a member here. It sucks. I hope it doesn't happen again.