I would call it a configuration choice instead.
Is the forum on an apex domain or not?
I always tell people that we are very cautious of setting headers that affect other hostnames on their domain, and if they want to have HSTS on those, they should set the headers on those respective hosts instead.
The only valid reason I can think of is they cannot do that, e.g. when the forum is on an apex domain and the client is not able to control the HSTS headers on other externally hosted hosts, e.g. they have hostedshopify.example.com as well. Then they basically come to you because you’re the path of least resistance 