I've seen it done before. In fact, when I first started designing, I did it as well without even knowing.
Illegal font usage. Just because a site provides free downloads to fonts doesn't mean they're free to use commercially. In fact, a large majority of them are for personal use only. Be sure to check!
The figure below is a screenshot from dafont.com, one of the more commonly used free font sites. Again, "free font site" does not mean the font is 100% no-strings-attached free. It simply means you don't have to pay to download.
I've circled points of importance in a minty green. Dafont.com gives you the option to filter the types of fonts you view by their license (see below). They also show which fonts are literally 100% free to use (personal or commercial), and which are obviously not.
Most "free" font sites provide this information one way or another. If you don't see it specifically stated anywhere, be safe. Automatically assume that all fonts have a separate commercial license regulation, and search for how to get commercial usage rights.
There are also some sites that ARE 100% free, both personal and commercial, such as fontsquirrel.com and Google Fonts.
This is no different than someone pirating your ebook, authors. Please be respectful to the creators of the fonts that make your cover art shine!
Rachel A Olson
Single mother of one, published author, southpaw, paranormal enthusiast, nerd/dork/geek/unicorn.