Pump and dumps are illegal because you are willfully attempting to manipulate the value of a security. And if you are doing this with other people as well, it's even worse because that implies it is more of a concentrated effort to do that. An example is if you and a few buddies are looking at a stock that only trades a few thousand shares a day. Between the group of you, you trade the stock so the volume is much, much higher than what it usually is. Others will notice this and think that the stock is going up by its own merits, clueless to the fact that you and your buddies are artificially driving up the price and volume. Others start buying and drive the price further based on the artificial demand you've created, and then you sell it off and the price probably tanks to where you started or less. In a sense and your buddies would be guilty of collusion and artificially manipulating the value of the security. That's why it's not fair, and understandably, that's why it's illegal. Furthermore, pump and dumps are also carried out by people that hack into others trading accounts, sell off their assets, and use the money to buy stocks involved in the pump and dump, which has the same effect of artificially driving up the price.
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