This is a good question, because it seems like a bit of a contradiction: these companies are there to make money and they’re spending money to develop these products but they’re still “giving” their code away for “free”.

Obviously I don’t know exactly what the thought process is by the executives of these companies, but I can take a guess. My guess is that open-source is great if you want to:

• Lower costs: get others in the open-source community to work for free in testing and spotting bugs in your software.
• Lower risk: this goes along with lowering costs i.e. lowering the costs lowers you investment risk. Furthermore, getting feedback from the open-source community is a lot quicker than having to wait for the software to be released and only thereafter get feedback from your users, by which time they might have created a negative impression with the end-users (if the software is full of bugs).
• Increase adoption rate: most of the products being open-sourced are SDKs and other development tools. This is very beneficial to developers using those development tools i.e. because it gives the developers more freedom and control. In the long term they end up with more developers/users hooked on their platforms at a quicker rate. As an example, I’m guessing it’s one of the reasons Microsoft opted to finally open-source .NET because it would get a lot more developers hooked on their platform, programming languages and development tools, which ultimately results in more apps being developed using Microsoft technology. Once they’ve increased the number of apps on the market developed using .NET, I’m pretty sure Microsoft have plans to capitalise on that in one way or another.