How is Montessori restrictive? Take the pink tower for example it is a self-correcting tower, it can only be built one way. If a child instead tries to lay them side by side they will be stopped and redirected to build it 'correctly'. How is this not restrictive?
The same can be said for all of the equipment used in a Montessori program.
Yes children are free to move about and to chose what activities to engage in and obviously this is very different from a traditional child care or school environment. However they are not free to interact with that environment in whatever manner they chose - that is how it is restrictive.
Edit - Ok just reread my last paragraph and appreciate that no school will let children interact with their environment without rules. But should they chose to use the pink tower to make a road, or to build a house then IMO they should be able to - the fact they are not is what is restrictive and rigid.