Only Water Sleeps has a new price. With further thought, my latest guess is that Tor is doing Print On Demand copies of the other three. I took a course this summer on the Publishing Industry, and learned a bit about the latest POD technology. There now exist machines that can print from a computer file mass market paperbacks that are about 80-90% as good in quality as conventional printing presses.
In the past few years Tor did POD editions of the early Barnaby Skye novels of Wheeler, whom I mentioned earlier. Those never made it to bookstores, but were only sold online by Tor themselves and Amazon/B&N. So I won't be surprised if these Black Company reprints never turn up in brick & mortar stores, while the omnibuses DO end up in stores.