If you read the itin carefully, I don't see how this is an illegal ticket, except if a US citizen actually tickets it. We don't know if it actually would have allowed the ticket to process with a US address, and I don't think the OP specified if it was or wasn't.
1. From the DR to France, so no embargo in force on that route
2. No US carrier segments, just CM and AF
3. Wouldn't this be the same as going on the website of a Central American or Mexican airline and doing a basic reward search to/from/through Cuba? Without being logged and having your citizenship on file, they have no way to tell where you are from to prevent you from flying there. I assume a lot of other carriers would let you book for $ or miles without checking. In all practicality, why would LAN, et al. bother setting up a system to check and block US citizens from booking somewhere in South America to Cuba? It is probably easier to just block US-HUB-Cuba routings as not valid, like the 'invalid destination' we saw on the first post.