Doesn't this DNR policy just serve to shield mediocre or poor providers? I think it could be used by scammers, fraudsters to shield themselves from just criticism.
Scammers and fraudsters won't be protected by DNR.
If service was mediocre but you can't say it was delivered in bad faith, I'd be inclined to respect SP's wishes if they were expressed before booking. Even without a formal PERB status.
It should be in her ad, or at least told during booking before you were committed.
There are valid reasons for DNR, and it's good PERB is recognizing it. There are potential clients who don't rely on reviews, maybe that's the best match for DNR SPs. Instead of doubting SPs' motives, maybe it's better to ask: "does she prefer to lose a client because of the DNR status, or would she agree to see a negative review if things go wrong." Her DNR request kind of answers that question.
The potential clients can choose, take a risk of seeing somebody with no feedback option; or pick another SP. I see it similarly to accepting other restrictions SPs may mention in their ads.