The supreme court granted MLB an exemption from anti-trust laws. The court then said that only congress can undo the exemption. Therefore congress meddles in baseball to make sure that it's still for the public good (and therefore can keep its monopoly). That's why they have the power to ensure the integrity of MLB. Like many things congress engages in, it's not in the constitution explicitly, but rather an interpreted power.
Don't get me wrong. I agree it's a waste of their time and they have better things to do. But, that's why they're involved.