I am just curious about it.
In history once it happened That GHASH.io owns 52% mining power, he was malicious and sell 5% to other miner. Bitcoin community keep watch on everything?
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Quite the contrary. If a pool becomes too big it just earns more money. There is no punishment. Maybe some bad reputation, but not something the majority will even notice.
It has happened three times so far that pools reached hashpower majority or got too close to it: Deepbit, BTC Guild and GHash.
Only Eleuthria, operator of BTC Guild, handled this in a responsible way. He raised the pool fees which caused some miners to react, moving to other pools. This balanced out the pools.
GHash scammed a casino, but they could have done this without having so much hashpower as we are talking about zero-confirmation transactions. They later said it was done by a rogue employee who is no longer with them.
Deepbit and GHash acted irresponsible and so did the miners. It appears that the vast majority of miners are not keeping watch on whether a pool gets too much hashpower or they just don't care.
The fourth time this happens, maybe some very bad things will happen. Hopefully in that case miners will take note and act more responsibly. It is doubtful though, as almost noone reacted to the casino scam mentioned above.
Disappointing for sure, but it is what it is.
There's no bad consequence in the protocol for having more than 50% of the hashrate. However, the community keeps a close eye on the pool repartition and will try to prevent such a pool to exist by moving to a smaller one (as it happened with Ghash.io).
Ultimately, the only punishment is a stronger community monitoring to try to prevent selfish mining, double spends, etc... If a miner above 50% stays malicious, the only other recourse could be a HF to another PoW function.