They make their money on very nice, very expensive professional control surfaces, cameras, and lots of other gear. Davinci Resolve is free because they realize funneling people into their ecosystem, having them learn how to use Resolve, instead of Avid/FCP/Lightworks/Premier/etc, which means that's what they're going to go with when they have the money to spend on gear. It'll just work better together. Like if you buy all Apple products instead of random company's stuff.
I'd think it also incentivizes Adobe et al. to make sure their software doesn't break the BlackMagic hardware, because there's a viable alternative (especially with the Studio version being included with the hardware).
