Naturally it would be preposterous for him to be on the ticket for President, but the reason they continue to let him run for Congress is because he counts towards the Dems in terms of determining which party has the majority and therefore sets the agenda for that chamber of Congress. Without him, that particular part of WV would almost certainly go to the GOP and tip control of the Senate firmly in their favor.
It’s a deal with a devil, and one that has burned the Dems more than once. But without him we’d probably be completely fucked.