Re: Why
yes, all of the above.
credit where credit is due. If he was a significant contributor to the invention, he deserves to be on the patent. If all he did was "what he was told", maybe not so much. But i expect there was actual engineering and creativity involved.
On the only patent application I was included on, I did the prototype implementation of the patented system. People barely associated with it (mostly managers) were also included, plus the guy whose idea it was (the engineering VP who briefed me on it so I could make the prototype).
So yeah, if this guy was involved in the creativity process, then he should be credited. All team members of significance in the process. What's wrong with doing that exactly?
(it's not like they have to pay him, they probably have 'work for hire' and other contract law on their side, but it's NOT a one-way street)