Agreed. However, the R programming language and many of its libraries are GPL licensed.
Can someone elaborate how R can be legally used in to develop proprietary or commercial software? RStudio's website [1] lists all of this large companies that apparently use R.
What you cannot do, however, is modify GPL software, as you're then creating a derivative work which must legally also bear the GPL license.
If you create a program that uses any of the GPL licensed R libraries, that program must also be GPL licensed [1].
Additionally, the GPL license prohibits using GPL licensed software in proprietary systems [2].
[1]: https://www.gnu.org/licenses/gpl-faq.html#IfLibraryIsGPL
[2]: https://www.gnu.org/licenses/gpl-faq.html#GPLInProprietarySy...