Because Allergan is facing several patent challenges and the Saint Regis Mohawk Tribe has sovereign immunity. That means they can file to dismiss these kinds of legal challenges which would make Allergan happy to have those off their plate.
What does the Tribe get in return?
The NY Times laid out the details:
“Under the deal, Allergan will pay the tribe $13.75 million and in exchange, the tribe will claim sovereign immunity as grounds to dismiss a patent challenge through the United States Patent and Trademark Office. The tribe will lease the patents back to Allergan, and will receive $15 million in annual royalties as long as the patents remain valid.”