Teaching materials like textbooks or work sheets can be opened up to free redistribution and free change for everyone. What risks does the publisher face, then deciding to do so? Are there legal consequences?
When open sourcing material, a commercial publisher faces the economical risk that anyone can offer the same material for a lower price. Another publisher would not have any cost for creating the content, only the cost of production, distribution and marketing (when they distribute electronically, the first two are very low). That would certainly allow them to underbid the original publisher and drive them off the market.
When the publisher has no economical interests, there is no particular risk in open-sourcing their work, except maybe hurting their reputation when the material is bad.
Possible legal consequences should be discussed with a lawyer who is familiar with the jurisdiction where the material is distributed.