Because they're using the "Private Use Area" of the font improperly...
Maybe I'm paranoid but I'm feeling like this is an intentional decision in order to push other fonts/foundries to start adding the Twitter logo as a standard Unicode character. Basically bypassing the need for consensus by just forcing the matter and cementing their logo in typography standards.
I don't think this will push fonts/foundaries to add the twitter logo any more than I feel pushed to use the twitter logo in handwriting. Fonts are very nice in that the user still enjoys a high level of choice in the matter. Heck, even the twitter nonsense could be fixed with a stylus/tampermoney script of one or two lines.
> Because they're using the "Private Use Area" of the font improperly...
Uh, isn't putting your own characters in Unicode's private use area specifically what that area is for? Isn't that where fonts that have dingbats unique to them, for instance, would put them?
Most companies based on user generated content try to claim ownership over it by way of agreements. That has always been murky legal territory and it's not easy to know what value they have.
If the proper rendering of content was made dependent on proprietary fonts, that could be one way to exercise control over where that content can be used.
It's not impossible that making users pick proprietary emoticons could have economic value. Remember where you heard it first...
Gut check: are there currently any trademarked symbols in Unicode? I can't think of any offhand, and it I find it difficult to imagine it happening for precisely that reason.
These are trademarked for other reasons than a company logo though, and I don't really see the point in including company logos (which might change or disappear tomorrow if they go bankrupt or are taken over by another company—and then what with the company logo in Unicode?).
Some Copyright is more Copyright than others...
This is essentially what happened with Arial w/ Helvetica; and companies have been doing this ever since to create new licenses that they can control.
We are drowning in Grotesk/Sans-Serif/Helvetica type fonts! Do we really need another "Big Tech Sans"? Or rather, why can't Twitter just use X? xD
Though to be fair, Depop uses GT America Extended and Expanded, rather than Standard.