They are not legal, but its not as simple as you think to get a website taken down, it takes time, i'm sure Mojang are trying, but for each one they take donw another pops up, just look at the sites for torrenting movies, thats illegal and yet they are everywhere and pop up when they are removed
Various countries have varying laws: what is illegal in one may be legal in another (ie. there are countries that do not recognize the concept of copyright).
Prosecution can only occur if a act can be shown to have taken place within a jurisdiction that defines that act as illegal. [Note this cuts both ways, many acts that are legal in some contries (buying beer, critcizing the government) are illegal in others.]
Even where the act may be illegal within a jurisdiction, law enforcement resources are typically spread thin. [Whether the solution is more cops or less laws is another debate.] Because of this, what crimes are investigated/prosecuted needs to be prioritized: things like murder, rape, arson, kidnapping, extortion,and robbery are going to be seen as coming before theft of a gaming account.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.