This misunderstanding arose from a deliberate attempt by the FSF to co-opt the open software movement by trying to relicense BSD work under GPL without seeking the author's permission. The attempt was successfully challenged by Theo de Raadt in 2007 (http://undeadly.org/cgi?action=article&sid=20070913014315) and the ensuing discussion in which Stallman was thoroughly educated in the law of copyright can be found here.
http://openbsd-archive.7691.n7.nabble.com/Real-men-don-t-att...
(A lot of that discussion concerned whether openbsd was open, but much revolved around the issue raised).
Most programmers are catching with the law in 2017, but the disinformation is still around.
Of course you can use BSD code inside a GPL project; but that does not entitle you to relicense the BSD code.