Distributors of restricted systems usually appeal to security concerns. They claim that if unapproved software can be used on the machines they sell, malware will run amok. By only allowing software they approve to run, they can protect us.
This claim ignores the fact that we need protection from them. We don’t want a machine that only runs software approved by them – our computers should always run only software approved by us. We may choose to trust someone else to help us make those approval decisions, but we should never be locked into that relationship by force of technological restriction or law. Software that enforces such restrictions is malware.
More details at the FSF website