Kiddie porn isn't the same thing. Kiddie porn isn't even in the same ******* galaxy as a tax return. Whomever sent out Trump's taxes broke the law, unless it was Trump or someone acting as his agent. Maddow and MSNBC did not pay for it and therefor as a journalist should could report on it without breaking the law. She also has Shield Laws that allow her to protect her source, though here those don't matter because the pages were mailed anonymously so she doesn't even know her source.
Both are illegal, one merely in the possession, the other to disseminate without the permission of the owner. Once they decided to release the information, they became part of the chain of release outside the limits imposed by the owner of said returns.
If the Tax returns actually showed something illegal, that would maybe give Maddow more of a shield from prosecution under the law, but the return shows nothing of the sort. How can she claim her actions were in the public interest when they show nothing but he paid taxes that year?