When we accept accept proprietary software, we lose our right to either one of these freedoms and so, proprietary software isn't acceptable for us that value freedom. Liberally licensed free software is acceptable as is but it also has the danger of being forked and transformed into proprietary software.
"proprietary software is bad! free software is good, but allows proprietary forks! let's make our own proprietary license, call it free, and spam it all over the internet until everyone accepts our twisted definition of free!"