Well, not "for free" to be precise. You may have noted that there is only one "Ext 6: Bugs" forum here — for both commercial and GPL versions. GPL users report bugs and suggest solutions there. The bug fixes then get released commercially only.
I am not saying that Sencha is owing those people 6.0.1 GPL — they report bugs voluntarily, and this hardly qualifies as "contribution". But now we have Sencha taking advantage of them for free, don't we?
If they are stealing there would be lawsuits. Can you point to some?
If you can't, the explanation is simple: Sencha does not sue them because Sencha knows — they do not actually violate GPL. There is a lot of myths and FUD around GPL, and this is exactly what the current Sencha GPL page is making on purpose.