The DNC's power comes from its ability to influence who the nominee will be. One the candidate has that locked up, the DNC loses most of its leverage, especially if there candidate will only serve one term, as Biden likely.
Nevertheless, let's assume that Biden truly is nothing more than a handpuppet for Tom Perez and his friends on the DNC. They have nothing to gain by pardoning Maxwell either. It would just poison their brand needlessly to protect a guy who has been out of the White House for 20 years and who doesn't represent the modern party anyway.
They stick together when it benefits them to do so. It's not absolute loyalty. For example, they circled the wagons for Ralph Northam and their rapist Lt. Governor, because Virginia can elect a Republican statewide under the right conditions such as an embarassing resignation. They had something to lose by throwing them under the bus.
However, they screwed over Al Franken and John Conyers. Why? Because there was nothing to gain by standing by them. Minnesota had a Democratic governor, who would surely appoint a Democrat to replace Franken. Conyers's district was so lopsidedly Democratic that they were sure to hold that seat. In fact, Conyers was friggin Ted Cruz compared to who holds that seat now (Rashida Tlaib).
Like Conyers and Franken, the Party gains nothing by protecting Clinton at this point. They would throw him under the bus in two seconds if not doing so would cost them something.
