I would lean towards probably not being petty.

Let's say somebody sued Honda because of an injury or death caused by a modified part that failed.

Honda wants to be able to say, "no, it was not caused by our part"
They don't want the lawyer to be able to come back with.........

"Is it not true that Honda condones even encourages use of this modified part in your Official Honda Fit Forum?"

1. They mainly just want to protect themselves.
2. +They are extremely guarded in the use of their trademarks.

 

I don't think that is really an issue, have a look at gigantic disclaimer the bottom of every single page. Perhaps guarded about the use of the name, but then again they advertise on this site too.
It just comes down to being petty in my opinion; somebody who has too much time at Honda to surf the web decided to make a big stink about it and draft a legal threat to FitFreak so he can feel like a big man.

 

Oh you mean the gigantic one in tiny blue type with the barely contrasting blue background? The one that says FF is a completely separate entity from Honda which right there keeps it from being anything "Official Honda...."
The one that also says what is on this forum is only opinion and fitfreak.net (let alone Honda) does not want to held liable in any way?

Car companies get sued for big bucks (10's of millions is not uncommon). They do not want the tiniest little thing to be able to sway a jury.

I know Unofficial is weird, but that is only because people who started this forum called it official.
Had they called it "the greatest Honda Fit Forum" there would not be an issue. But then if Honda strong armed FF to change it to "the ungreatest...."
now that would be petty and I would be pissed
The rebel outlaw factor of "unofficial" is cool in a way as I drive a wimpy tiny car that is THE anti huge SUV of cars.