Using a font that came with a Mac App Store app in other apps I'm sort of generally curious about this, since pre-Mac App Store apps one could simply open the font file from within the app's package contents and install it through Font Book to allow use outside that particular app.

When I use Add Fonts from within Font Book to add the fonts, I get the following 2 "serious errors":


  Corrupt font name
  - Font basic parsability
  - System Validation


Now, is this something I'm just not supposed to be doing? As in, am I breaking a rule here? I'm not using these fonts for publication or commercial use - I just want them available throughout the system.
In the case of iA Writer, they licensed a commercial font called Nitti Light for use in their application and their application only. That's why you cannot copy it into FontBook and use it elsewhere. Try Cousine from Google Fonts for a free font that is quite similar.