Franz Only reason I can think of is for spammers to target flarum if they want to, or if there's an unknown bug and script kiddies do a google search to wreck random havoc.
Basically, putting "powered by flarum" on default on every page is the same as ringing a dinner bell for spammers who target discussion forums, regardless what powers them. They want forums.
This is why free forum solutions like phpBB and SMF are targeted.
I suggest an "about" page with the current staff and mods listed, what the forum is about (admin CP configurable) and a single line that it is powered by flarum. Then it can be set where bots don't crawl that page. Or even a credits page with the same "no bots" rule where you can detail more into flarum info without a small line in the footer of every page that can be used against the forum itself.