That's a cunts trick! I see they also appear to schedule certain girls with a (TBC) caveat. A bit of false advertising to bait and switch in my opinion but might be a flaky prossie. If the later then they should take her off the rota.
I was tempted to book one of two recently but with the negative reviews and now the OP's experience then I am avoiding like the plague!
Looking at the names involved, they are the ones who have shown a propensity for unreliability, often coming up with flaky excuses on the day for not turning out of bed. I suppose they put TBC so that guys booking them know that if the book in advance it's a coin toss whether they'll actually show. I think repeated unreliable behaviour does get the boot, but some of these girls are very popular and great providers when they do show, so the TBC label is an attempt to instill discipline.
HoD places may have made the odd mistake on the rota in the past, but I have no evidence that they practise bait & switch.