This is not completely true, in PoW networks those who controls the majority of the hash power will be able to skip some transactions and prevent them for being added as an update of the ledger status...
What prevents this from happening on Bitcoin is the economy cost associated with such an attack, and this economic cost is high because of the value the Bitcoin network is high... Justin was able to pull all of this out because of the stupid price Steem has, if Steem was decently priced 2ish$ I doubt he would dare to endanger the credibility of the network with these actions... Because for a man that pays millions to have a lunch with someone, messing with a 70M$ worth network is like playing with a backyard toy.