I don't agree with that and I don't understand why those charges were not taken to court (I don't believe it was purely because he already had 2 life sentences as when the appeals were going to the supreme court and there was a risk of him winning, it surely made sense to secure him if you had the evidence to proceed with another case). Whether that was true or not though, someone can only be sentenced and punished on what one was convicted of. Anything else is not based in justice or law.
I don't really believe that the US is a country ruled by law any more, but in some idealized land of the law you are probably correct. But you won't convince me that OJ Simpson walking on a double homicide he clearly committed is justice. For Ulbricht, it actually feels like things more or less balanced out, but he did try to have someone killed, and I don't think that's a nice thing to do.