One could take the view that a full TLS stack is so complicated that it is much more likely to contain exploitable flaws than some simple signature checking code.
Maybe people hand-roll their own signature validation code badly, but those same people will just as much screw up or plain disable the CA verification.
If you have the knowledge to use the primitives from something like NaCl, you have nothing to gain from using a full TLS stack but pain building, upgrading, programming your firmware and massive middleware problems in the field. And when they inevitably find issues in the TLS stack you used, your device is now fucked since there is no virtual address space, W^X, even stack cookies..