Edit: the below isn't a very good answer to the issue; I will leave that anyway. But it boils down to that in order to divert responsibility in a big company, you have to show you did all you could and to do that all you could, you need to show you paid money to 'the head ninja'. This could be MS/Oracle/IBM: very safe. When using Open Source, you need to show as well that you did all you could when it fails (if it succeeds, the company wants to be in the news using open source to 'cut costs') and so you pay people who can support it (the original devs) large amounts of money to support you. TL;DR It's covering your ass.
Previous reply;
You haven't worked with big companies before then; they pay for support (and licenses if needed). It's how it works as it is a liability thing; if you pay for something it's better than free as free doesn't exist. And that's kind of true; for free open source you need highly skilled employees to maintain it while you can also buy support from the company who actually built it. TL;DR big companies buy your support if they use your open source.
Previous reply;
You haven't worked with big companies before then; they pay for support (and licenses if needed). It's how it works as it is a liability thing; if you pay for something it's better than free as free doesn't exist. And that's kind of true; for free open source you need highly skilled employees to maintain it while you can also buy support from the company who actually built it. TL;DR big companies buy your support if they use your open source.