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Still, it seems weird that Congress wouldn't get a say in determining what their own bills mean when there's a dispute.That's an interesting point that has me thinking.
My feeling on this is that this is actually as designed. Congress' job is to pass laws after writing them in whatever way they deem prudent. But Congress doesn't really have an interest in the enforcement of those laws, up until the point their constituents come to them and say "hey, this law y'all passed, it isn't really working out... do something to make it better". And if there's a dispute as to the enforcement of those laws, an independent third party (the judiciary) mediates and decides, as seems appropriate. But, again, Congress doesn't get involved there, because it's the executive's job to actually apply the law.
If Congress does get a say in interpretation, that means they could change their minds on existing law as political winds change, without going through the proper process of passing new legislation. They shouldn't get to do that, especially not for laws that are already on the books. That would amount to the power to write ex post facto laws, which are considered such a bad idea that they're expressly forbidden by the US Constitution. I think the risk of passing a law that gets interpreted in a wildly different way than Congress' intent isn't that high, and in cases where it does happen, it's usually because culture has fundamentally changed over time. Congress' remedy of passing a new law, but one that will only cover future cases, is a fair trade off, I think. And in cases where there's a big screw-up, I'd expect it would get noticed fairly quickly once cases start hitting the courts, so Congress would have the opportunity to fix things, but only have a short window of time where unintended consequences happen.