this simply is not true at all...

in a PoW coin you have to convince the miners to adopt the new fork.
If 51% of the miners do not agree then the fork simply does not happen.

due to its size and also its PoW implementation it would be virtually impossible for the bitcoin devs to do something like this with bitcoin without first gaining the consensus from mining community.

miners play a vital role in securing the network not only from outside attacks but from internal misuse of power. they are separate from stake holders with separate interests so they act as a counterbalance.

in a PoS coin there are no miners so there is no separation of powers.
in theory stake holders should play that vital role of making sure that power is not abused.
in practice however.. stake holders do what they are told or get locked out of the system.

this is not just a problem for PoS coins, it does affect all small growing coins, the PoS part merely amplifies problem because some crucial checks and balances are removed.

so I reiterate that it really isn't true at all so say that ANY coin can be rolled back. Some are many orders of magnitude more difficult to roll back than others.

one needs to also consider that rolling the blockchain back isn't just invalidating 1 or two transactions.. it is invalidation every transaction across the entire system that happened after the roll back point.
when people are using the network to buy and sell goods you simply cannot do such a thing without causing major financial damage to the people using the system.

in the case of a small alt coin where the majority of trading is happening off the block chain at exchanges and being done by speculators then there isn't such a big issue with rolling back.