I've been having trouble with the 'open' definition & list and the connection to camera preference. I may be dating myself, but to me the definition of "open" is relative to "closed". Just like, in the early days of the PC, buying into Apple's iOS means you were stuck with their iOS ecosystem. If you went with the Win/Intel architecture, you could expand or customize a system with all sorts of other makers' components. (In our business, I believe the choice of server Operating System is moot - the discussion is really about which VMS application software is 'open' or 'closed' to which cameras, and vice versa)
One example of a 'closed' or 'locked' camera system would be DVTel, because their cameras only work with their software. If you install them, you're stuck with their VMS -- it's a one way street, however, because the DVTel VMS works with other cameras. As a customer grows their system, every DVTel camera tightens the handcuffs against leaving for another VMS. I've not used Mobotix, but understand that it is largely a two-way lock, with proprietary hardware and software requiring each other to form a system.
To me, Canon now owning Milestone may make that VMS more 'closed' over time if it grows to preferentially support Canon cameras, but they're still currently 'open' in the sense of interoperability. Their acquisition seems more a play to get Canon hardware into the surveillance business, either by offering bundled pricing with Milestone licenses or, as Glenn Fletcher suggests above, by opening up the specs and secrets that other camera-makers have shared with MS up until now. I wouldn't blame those brands if they choose to cinch their Kimonos a little tighter now that they know Milestone's parent will have visibility to their privates. Over time, Milestone's loss of market share will likely follow the decline of preferential 'market-leader' treatment by Axis and other camera makers. I expect this decline will be faster than any growth of revenue that Canon hopes to see in the surveillance hardware marketplace.