Some companies (and individuals) choose not to directly (or indirectly) promote other companies and/or products; so they pay the extra to remove the branding and, most importantly, the links back to the software maker.
SONY, for example, uses XenForo for all its community gaming forums - https://www.soe.com/community.vm - and I would imagine the brand managers took one look at the copyright footer link back to XF and said "That's gotta go!!"
I got it because the less clutter on my site, the better.
Kind of off-topic, but I also removed a ton of redundant things... for example on the navigation tab drop-down menus... is it really useful to make the menu twice as big with a giant, "Members - Quick Links" label when I clicked the Members tab to show it? 1. I know it's the members menu because of how I activated it. 2. What makes them "Quick Links"? I know it quickly showed because I witnessed it when I activated it.
So yeah... for ME, the less clutter/redundant stuff, the better.
Surprisingly, not everyone yet knows what XenForo is. And if you do some tweaking, you can basically remove the xenforo name everywhere (even rename xenforo.js). So the end result is your competitor is scratching their heads wondering how on earth can they mimic that cool site.
Its funny, because most branded free forums are easily identifiable; maybe not everyone can tell, but I figure most can. The Sony forums for example, easily can tell they are xenForo. There was one website forum I came across that was so heavily modified I couldn't even tell it was vBulletin v3, until I read the source code; the branding was removed, and I admit that was pretty nifty - almost figured it was custom coded.