On a related note (I can't speak for Dropbox specifically) there are many engineers who desire their work to be open source for their own motivations, and when it's not a significant business risk to do so, nice companies will allow it.
So it works on two fronts, as a "hey, we'll let you open source your stuff" and a "hey, we've got people here who care about and contribute to open source."
In google there is a regret that they didn't open source a lot of stuff, because it ends up being reproduced outside of google in some form. The open source companies have an advantage in hiring and overall advancement of their product by using the open source version whatever their internal version was.
You also see it in facebook's open source initiatives, like react, buck, haxe and so on.
What regret? A lot of the open-source clones of Google's internal projects come from Google themselves.
It's interesting that they don't consider it a large enough competitive advantage.
Either that or they are using this to attract engineers.