Tk Source Code

It looks like the reason it's there is that whoever did it thought the fonts were ugly, from the comments? That seems a really bad idea to block access to those specific fonts at that level, especially since they're not blocked from other places in Tk (such as the built-in Windows font dialog, so people can still try and select it should they wish to and then end up with what seems to be a bug). 

Back in 1999, Courier at least was likely the crappy version that pixelated, not a proper truetype font (though I've seen plenty of perfectly good fonts called Times and Helvetica on different Windows machines over the years).

This just seems really arbitrary, and something much better left to individual program writers to me.