Some things should be illegal even if they are otherwise taxable, or even if they might go underground. We should not act in fear of things going underground. We should stand for good principles and let the cards lie where they call and use police to stop crime. If we don’t like things going underground, we should lobby for a stronger police force, not for weaker standards. This is what Moroni did when threatened by the “king-men”.
The church is oft against regulated legal evil as seen in their opposition to prop 8 (legalizing homosexual marriage), and Elder Quinton L. Cooks devotional a few weeks about “the banquet of all choices” (see speech at https://speeches.byu.edu/talks/quentin-l-cook_banquet-consequences-cumulative-result-choices/) speaks about how the movement to legalize marijuana for recreational use is evil.
In the same logic of legalizing drugs etc. so they don’t go underground and give the bad guys the tax benefits, we may as well call for legalizing other bad activities that “are going to happen anyway” in the name of getting taxes to the US Government. We could legalize rape, child abuse, child pornography, prostitution, pornography, murder, etc. We may as well elect the Devil himself as President to not offend anyone! Surely, he will do a good job at representing the 1/3 minority… You see, some want to legalize these things so things that are already happening can be further regulated. But then we become the bad guy. This money is dirty money, and I want nothing to do with it! We want to get rid of drug cartels yes, but not by becoming the cartel ourselves! The USA needs to take a moral high ground, and not sink to the depravity of evil. You see, the ends to not justify the means!