I've noticed that most custom fonts include a license.txt file that requests some amount of money for a commercial license.
I am interested in whether it is also necessary to license OS (Mac/Windows) included fonts for commercial use. As a side question, is font theft an issue in the industry?
I am interested in whether it is also necessary to license OS (Mac/Windows) included fonts for commercial use.
Depends how you define "use". IANAL, but as far as I know, using a font you legally purchased (either separately or as a part of the OS) to create creative work, and redistributing the font as part of that work in a non-embedded form (i.e. as a bitmap or with the vectorized outlines of the letters) is always okay. Meaning that if you use a font to e.g. create a logo, the font's creator will usually have no rights to the logo. (See @e100's comment for an example that says otherwise. Be sure to always check the actual license.)
What is usually out of bounds, or limited, is redistribution of the original font "software" - including converting the font to another format, embedding it in a document, or in a web page using
If in doubt, check out the font license that is supposed to be shipped with every font... Although I have no clue where that license is supposed to be e.g. in MS Windows. Maybe you'll have to contact Microsoft and explicitly request it.
Fonts shipped with Open Source OS's usually come with a less restrictive Open-Source license - see this question for a list. But note that the GPL may also inherently prohibit the redistribution in closed-source products under some conditions - check the license carefully before including the font in any product you sell.