GPL means Free as in Freedom
Not as in Free Beer
Plugins, by definition, use WordPress GPL codebase.
They are considered derivatives.
So are themes.
2011 | 2012 | 2013 | 2014 | |
---|---|---|---|---|
Investments | $398M | $669M | $920M | $1.3B |
Average Deal | $8M | $14M | $18M | $33.9M |
Num. deals | 49 | 49 | 51 | 39 |
Source: 2015 Future of Open Source Survey
Open Source and GPL are what make WordPress so great!