After being on the inside, I am convinced this is (part of) the reason why Microsoft seems so disconnected from the rest of the industry: no one here reads anyone else's code.
Sure, you might look at other code in your (probably enormous) project. And if you can swing it, you might be able to look at another group's code. But in general you can't go look at, say, random third party open-sourced code similar to what you're working on, or even better, random code that has nothing to do with what you're working on. (The license, viral or not, usually has no effect on the prohibition.)
There are aspects of serendipity, and of feeling recharged by feeling connected to the greater constellation of developers and projects in the world, and those aspects are stripped away here. I wonder if the Microsoft "culture" attracts people who don't care about reading others' code, or if it creates those kinds of people.
Contributing to Microsoft-owned open source projects is acceptable, in some cases. A small number of products are run in an open-source manner within Microsoft, and actively recruit contributers inside the company.
I don't want to talk too much about internal policy -- I'm sure the legal office (LCA) wouldn't appreciate me pontificating about how cracked-out I think their opinions on F/OSS are -- but in general, 'forbidden' would be a good description, both for contributing and just reading code. There are exceptions, but they have to go up several levels of management and be directly work-related to be approved. (I think MSR is a different world, though.)
Honestly, most devs I know are too slammed with work as it is to keep up with a large-scale F/OSS project after hours. Every once in a while you get enough of a breather+motivation do small hobby projects and keep up with new technology.
The F/OSS restriction is the #1 thing I don't like about my job, but honestly, it's about the only thing.
Sure, you might look at other code in your (probably enormous) project. And if you can swing it, you might be able to look at another group's code. But in general you can't go look at, say, random third party open-sourced code similar to what you're working on, or even better, random code that has nothing to do with what you're working on. (The license, viral or not, usually has no effect on the prohibition.)
There are aspects of serendipity, and of feeling recharged by feeling connected to the greater constellation of developers and projects in the world, and those aspects are stripped away here. I wonder if the Microsoft "culture" attracts people who don't care about reading others' code, or if it creates those kinds of people.