@Jeffro - That's the way I see it, too. The client-developer relationship is one in which the client provides the developer with access to the work under his own authority and only his own authority to have/use that work, and with the explicit intent that the developer will return to the client both the work and the authority to use it at the conclusion of the business arrangement.

Should the developer make any other use of the work during/after that business relationship, it is the developer who is stealing the work, rather than the client having illegally distributed the work.

So, yes, there might still be a concern on the part of the copyrighted work's developer in such a scenario, but that concern should be centered around the client's hired developer, not the client himself.

(Note that such risk is not unique to theme development specifically or software development in general. Holders of copyrighted works and other IP have to manage this risk in pretty much every industry and line of work.)