Unrestrictive things are loose, like sweatpants, and they don't come with a bunch of rules or fine print (no "restrictions apply").
An unrestrictive environment is one that doesn't come with a lot of limitations — it's "lax," "unsupervised," or "unstructured." This might be okay for free time, free association, or free jazz, but it probably isn't such a great idea when you're teaching, say, rocket science, electrical wiring, or space shuttle navigation.
