If someone has wrote "open source", it is because they mean it. If they had wrote "free software" then it might be ambiguous, but nobody ask for "open source" software without having at least a basic understanding of what that means and how it differs from "gratis".
It is a requirement they choose, so I don't think that telling them to broaden their requirements is generally useful.
In the case of Open-source mail merge software:
- The question was already too broad, as "merge" can mean pretty much anything. For instance, any email client like Thunderbird is able to merge mailboxes, you just have to import the two mailboxes, and CTRL-A,CTRL-X,CTRL-V the first into the second. The question was already too broad (as well as unclear), so there is no need to broaden its license requirement. Once the question is made less broad by clarifying exactly what kind of merge, then a broadening of the license requirement may be considered if the question is ultra-specific and has no open source solution (which I doubt)
- In addition to writing "open-source" in the title, in the question body, and as a tag, the asker took the time to write this in a comment: "Looking at what exists, I had trouble finding any FOSS software that would have [XYZ] functions". When someone is specifically looking for open source, there is no point asking them to modify their question to include non-open source.
There are dozens of reasons for using open source. Forking and audit are only two of them. Maybe they work in a company that has an open source policy, maybe they plan to redistribute their solution, maybe they have a philosophical reason, maybe they are tired of nags or privacy invasions that often come with shareware/freemium, or something else. It is not our role to try and guess whether they really need open source, nor disbelieve them if they think they do.