[legitster]

If they pick a font, even an open source one, it could encourage people to copy them. Or be making some (very minor) statement about open source.

By developing their own font, they don't set preference for any organization, group, or business.

[snazz]

They do develop it on GitHub, which I would consider to be a stronger statement than font choice.

[beatgammit]

Well, nobody said they had to be consistent...