The OSI, and Open Source Movement in general, views proprietary software as having its place, and able to co-exist with FOSS, and advocates for FOSS as a matter of pragmatism rather than ethics. One example of this type of pragmatic argument is the essay The Cathedral and the Bazaar.
As a movement created from and in response to the FSF's antagonism towards proprietary software, OSI deliberately positioned themselves as business-friendly. The most obvious example of this position is in the labelling itself: Open Source, compared to Free Software, doesn't imply free-as-in-beer and anti-commercialism; while FSF is well aware of this problem, the people behind the Open Source Movement felt it was a big enough one to adopt an entirely different label.
The OSI website once had pages explaining all this. It doesn't mention proprietary software directly but its position is clear if you read between the lines:
How is "open source" related to "free software"?
The Open Source Initiative is a marketing program for free software. It's a pitch for "free software" on solid pragmatic grounds rather than ideological tub-thumping. The winning substance has not changed, the losing attitude and symbolism have. See the discussion of marketing for hackers for more.
The Marketing Case – New Territory for Techies
The case that needs to be made to most techies isn't about the concept of open source, but the name. Why not call it, as we traditionally have, free software?
One direct reason is that the term "free software" is easily misunderstood in ways that lead to conflict. You can read an extended discussion of this problem.
But the real reason for the re-labeling is a marketing one. We're trying to pitch our concept to the corporate world now. We have a winning product, but our positioning, in the past, has been awful. The term "free software" has been misunderstood by business persons, who mistake the desire to share with anti-commercialism, or worse, theft.
Mainstream corporate CEOs and CTOs will never buy "free software." But if we take the very same tradition, the same people, and the same free-software licenses and change the label to "open source" – that, they'll buy.
Some hackers find this hard to believe, but that's because they're techies who think in concrete, substantial terms and don't understand how important image is when you're selling something.
In marketing, appearance is reality. The appearance that we're willing to climb down off the barricades and work with the corporate world counts for as much as the reality of our behavior, our convictions, and our software.