Suppose he is ... so what !
At the end of the day, it's just a job. The country angle is BS. If he is getting undue stress from the job (mental or physical) and is not too interested in it then he has every right to move out of it. No one was doing him any Favors when he was picked up for the national team. He was picked up purely on SKILL. Therefore, he owes no one and there is nothing ungrateful about his decision. He just wants to play RELAXED cricket and enjoy the game instead of dealing with all the burden that comes with leading national team's batting lineup.
Now if suddenly, God forbid, there is a dire need for Stuart Binny or Rohan Gavaskar in the national team, and they say NO to it, that would amount to ungratefulness. Because they got in due to contacts and not skills, and owe it to the system.