This is a problem at any stake. Random coalitions to alter the past can be formed at no cost to those colluding.
Checkpointing is not an alternative to decentralized consensus but central override of it.
The same works for Bitcoin, too.
You misunderstand. The risk isn't that someone could attack the network, it is that they could attack the network with
no cost.
Imagine bitcoin worked using a PoS. An early adopter had acquired 1M BTC at one time in the past but over time he lost/sold/spent/transferred them. Today he has no bitcoins but the blockchain contains a history of a time when he did have 1M BTC. If the amount of the stake being used is <1M BTC he could rewrite history not by using coins he has today (a real cost), not by buying millions of mining rigs (a real cost) but by using the history of the coins he once had (no cost). He has absolutely nothing at risk and nothing to lose. If he and potentially others decided to attack the network they would rewrite the blockchain starting from when they had a larger stake, creating a parallel history where they didn't lose/sell/spend/transfer the coins.
They can attack the network based on what they had (but no longer do) in the past. There is nothing at risk and no cost to the attack. THAT is the PoS problem.
If bitcoin miners collude, they could alter the past.
Sure they can, however there is a cost to that attack and there is something at risk which they lose if they fail. With PoS you can attack the network for "free" using something you had but no longer do. It is very hard to secure against an attack where the attacker can do so at any time without any cost and without any risk.
To POS mine you have to have the coins now in your possession. The fact that you have spent them [according to your senerio] means you can not longer mine with them in a proper POS setup. Eg, spent coins, for you can not anymore accumulate coin age.
Thus your mining power = 0.
on another point, NXT appears to be 100% POS and has not been forked or hacked by anyone to date. Further it Appears that the network swiftly punishes miner that try to undertake dubious activity, like producing dogey blocks.