I don't think Sonance or Polk directly mention back-boxes in the sales literature for their in-wall/in-ceiling speakers either, but they sell them as accessories. There are probably other brands they go to the trouble as well. I expect most in-wall/in-ceiling drivers would benefit from a properly sized enclosure, but the percentage of people who would actually install them is small so most mfgs. don't bother.
Anectdotal data point. A few years ago, I installed a pair of HiFi Works (division of Sonance) THX certified in-wall surrounds. I emailed Sonance/Hifi works for the recommended back-box volume. After some use without an enclosure, I went ahead and boxed in the joist area around the speakers to the recommended volume. It made a significant difference in the bass response of the speakers. Unfortunately, I didn't take any before/after measurements so I can't quantify the actual improvement. IIRC, the enclosure dropped the Audyssey recommended xover point 20Hz or more.
Similarly, I've since picked up another a pair of the HiFi Works THX LCRs, although theyr'e not installed yet (future project). The installation manual for these actually recommends assembling a back-box around them for best results. And, surprisingly, since one would expect in-wall/in-ceiling woofers to be optimized for infinite baffle operation, HiFi Works recommends only .26 ft^3 for these...rather petite by IB standards, IMHO.