The GPL doesn't force people to upstream their changes, although it often has that affect. The GPL only forces you to give source downstream. If your customers never share the source with anyone else (and your upstream is not one of your customers) then your upstream will never get the changes. A good example of this is the game TOME. It has downloadable content that is licensed under the GPL. You get the source code when you buy the DLC. I've never seen anyone distribute it, though (and it's highly frowned upon in the community). The author has a weird idea of the GPL, though, so I don't think he really understands that anyone is allowed to distribute that code.

But in practice people usually freely distribute GPL code, so it's impossible to stop your upstream from eventually getting it.

it would be somewhat counterproductive to introduce yet another sandboxing mechanism, just to work around a problem created by upstream - especially in case of mechanism as awesome as Capsicum :-)

Yes, the GPL license would force them to share their changes. Thing is, they wanted to upstream them anyway - AFAIK the problem is on the other (accepting) side.