If engineers are the most important resource, I think being a company known for open source might act as an inbound sales for attracting top talent, too (see: Google, RedHat, Mozilla). For engineers, it makes sense to work for companies that do have FOSS projects because they can always point to their corpus of work when switching between jobs as proof of their worth. Most engineers who realise this don't ever want t…
For instance, I know people that work for heroku that work on open source (and are primary maintainers in some case) on company time -- but heroku is obviously not a FOSS company.
Or, in a comment below jedberg says 'At Netflix our ethos was "if it's involved in infrastructure, open source it, if it's related to movies, keep it closed.' -- nobody would call netflix a "FOSS company".
In the original wave of open source, this was the position of most contributors to open source I think. Work on a thing on company time that helps the company achieve it's goals/business, but is not the main focus of the company.
This seems to be less and less the case. Because companies are more ruthlessly focused on the bottom line, and don't think they can afford to have anyone working on something they don't own or that might help a competitor? I dunno.
But this change has a lot to do with the current apparent sustainability problems in open source.