The ESRB does not rate the DLC but the game already has it's rating. Sure, they could turn off the filter, but if you knew anything about Harmonix and their fanbase and target market for the Lego game, you would know that the filter has no chance of ever being disabled. Get over it, it's not going to happen.
Kario Charts - Charting Uncharted Territory
Yeah, I would say this probably has to do with the LEGO license and therefore, I would never expect the filter to be disabled.
That being said, it does seem a little haphazard. It would be nice to see some songs get reviewed and OK'ed. (Seriously? Don't Stop Believing isn't allowed?)
Yes a song about sex and a smoky bar is not allowed.