I’d imagine precisely because they might be collecting regexes from user input such as parameter values or search terms, and the user may not know or care which technology your tool or service is built with. However, they will need to know which regex dialect(s) you support.

And I’d further bet that people who are casual about specifying that are relatively strongly correlated with people who are casual about santization, catastrophic backtracking, etc. (At least based on code I’ve seen over the decades.)