They aren't entirely invalid, though interestingly the only times something like this has ever happened were around that Java platform, not .NET. And yet, even though they're the ones who are notable for actually trying to do so, nobody worries too much about trusting Oracle not to try and torpedo any open source ecosystems for profit.

Which is why I'm inclined to think of it as FUD-spreading. It's not about a balanced assessment of the strategic risks involved in using a particular open source product so much as it's about how one company in particular has been mythologized into something approaching a Lovecraftian horror in the minds of many members of a particular subculture.