"class legal" implies Hobie sanctioned events. For non Hobie sanctioned races you can use a sail from any sailmaker.
Not entirely correct.
The International Hobie 17 is a one-design class. Part of the "buy-in" to a one-design class is that everybody plays by the same rules insofar as equipment is concerned. The theory being that the skill of the sailor and not the equipment (or the depth of their wallet) determines the results on the water.
One of the Hobie 17 rules is that class legal sails come from Hobie Cat (as it is for all of the Hobie classes). Byproducts of this rule are that 1) everyone has a sail cut from the same patterns and 2) it helps support a robust dealer network that continues to support boats no longer in production.
Handicap ratings are determined based on the class legal configuration of a boat. If you have a non-class legal sail, you will take a rating hit under every catamaran handicapping system in use - because the boat is not class legal.