Having some proprietary code doesn't prevent companies from also making substantial contributions to open source in other areas.
... yes, but patents are not "proprietary code", in which case granting copyright is enough. If a private company contributes code I think it could even grant the copyright to an open organization, but still decide later to sue if the method/feature/function/etc is patented by them.
Not that I'd foresee IBM or any of the real players in the IT space attempting that anymore, I think most realize that alienating the F/OSS community isn't a viable strategy for a software services company in the long term.
Not that I'd foresee IBM or any of the real players in the IT space attempting that anymore, I think most realize that alienating the F/OSS community isn't a viable strategy for a software services company in the long term.