WPMUDev is 100% permitted to do what they're doing.
They're not taking other people's code, they're forking (or it's being written for them, they don't steal without attribution knowingly, they had an issue with that once and IIRC fired the dev in question).
They're charging for the service of their support and products, which is totally permitted, and actually encouraged. If you find their plugins to be of value, or not, doesn't change that. You're welcome to use them, or not, as you see fit.
You can download plugins here: http://wordpress.org/extend/plugins/
Be aware not all are free! All are free to download, but some charge you to use their APIs (i.e. if they are processing code on their servers) and others have paid upgrades, or paid support. All of those methodologies are allowed. You may have noticed WPMUDev has free versions of their plugins here :)
However. None of this has anything to do with Multisite, so I'm closing this post :)