The simplest standard of legal font use is that no one is allowed to use a font on their computer unless they specifically have a license for that font . When a font is ‘purchased’ the user never really owns the font-they typically receive a license to use that font on only one computer. These End User License Agreements ( EULA s) differ between companies but generally state quite clearly that the fonts may only be used on machines for which there is a valid license. These licenses can be ‘purchased’ in a variety of ways: directly from the font vendor (Adobe, Bitstream), as part of a larger software package (such as Microsoft Office), or just by downloading the font from a web site (as with most freeware fonts including many SIL fonts).
However the font is obtained, almost all licenses do not allow the font to be passed onto others. So the most basic legal rule of font copyright is that unless the license specifically allows it, fonts cannot be shared among multiple computers, even if they are all owned by the same person or corporation, and fonts cannot be given away to others. In all cases the EULA is the authoritative source for specific licensing details.
The basis for this is that when you obtain a font (or any other software) you are making a legal agreement to abide by the EULA . If the conditions stated in the EULA are broken then the other party has a right to claim any damages as a result of that broken agreement.
Cited from: http://scripts.sil.org/cms/scripts/page.php?item_id=UNESCO_Font_Lic