There are two freedoms of different people (or rather different roles) that are in conflict here: the freedom of developers to do whatever they would like to do with code that they have access to, and the freedom of users to be able to change and control the software they use. MIT/BSD/etc prioritise the former, while GPL prioritises the latter: free software advocates generally believe that proprietary software is immoral, and that _all_ software should be open to users to modify, even if that limits developers freedom to keep it secret. The GPL is an attempt to enforce this as much as can be achieved under current law, not a natural reflection of their wishes (which would compel all software to have source code available for modification).

(There's also a secondary motiviation for using the GPL which seems to be driving this kind of discussion, that of 'paying your part', but this is neither open source nor free software in origin IMO: the desired deal is 'I make the source code available, but I want a cut if you're making money from it'. You can often do this with a version of the GPL that is viewed as sufficiently anti-commercial, and then offering a paid proprietary license, but this is antithetical to the goals of those who wrote the GPL)