So to summarize, if a credible attacker announces his/her intention to destroy a POS currency, holders of that currency are likely to sell to the attacker very cheaply due to their belief that the attack will be successful and thus their coins will be worth nothing if they sell them. Very interesting point. In fact, it presents a "prisoner's dilemma" of sorts, where an attacker can be thwarted if the majority is able to resist the pressure, but where it's in the interests of an individual to break consensus and deal with the attacker.

The attack has no cost or risk.  

Very simplified example:
The network stake is 2M xCoins.
I acquired 1.1M xCoins as of block 1,000.
I sell you 1.1 M xCoins for $$$$$$$ and the transfer is recorded in block 1,001.
I now no longer have any xCoins (effective block 1,001+), I have no cost as I received $$$$$$ in return for the 1.1M xCoins.
I start building an attack chain as of block 1,000 double spending my transfer.

Eventually even if the main chain has a head start, my attack chain will be longer.  This is no different than a 51% attack on a PoW based network however my attack has no cost and no risk.   I already sold the coins.  I am merely using my history of prior ownership to attack the network.

Compare that to PoW.  I build a hashing farm with 51% of network capacity.  If I attack with it then the attack has cost and risk.  The farm wasn't free, I may not succeed in which case I would lose all the legit blocks I could build.   If I sell the hashing farm I can't engage in an attack based on the history that at one point in the past I had more hashing power than the rest of the network.

Both are vulnerable to a 51% attack however PoS allows the attacker to exploit the history (your security mechanism is recorded in the very thing you are attempting to secure) to attack without cost or risk.