The problem, if one want to trade crypto currencies, is that you have to record every transaction. Then one has to figure out if the coins you have traded have been held for over a year and thus treated at the long term capital gains tax rate as opposed to the short term rate. Then, I believe with the new tax bill, one has to, going forward, sell the oldest coins first (if they are going to be treated like stocks). This makes complying very hard; impossible if you did not start at the beginning being very careful with your records. Basically trading a lot, especially algorithmic trading, is not something illegal, just impossible to do legally, in practice. Same with stocks. Professionals (of course) have some kind of special rules where they only have to keep track of net gains (or losses) and not every trade.
A good analogy would be if, when gambling at a casino, one had to keep track of every bet and report it and the outcome. Then, if one kept chips for over a year, you would also have to keep track of how old the chips you were betting were. Cashing out chips you held for longer than a year would have a lower tax rate. New rules now would say you have to bet with your oldest chips first. Now report all this activity to the IRS with your taxes each year. Vegas and other casinos would not exist with such rules and so much pressure would be against laws treating Vegas style gambling the same way.