This is why Casper tries to punish those that do not follow the rule ; but then this can be gamed too and you get a hopelessly complicated game-theoretical bungle.
How can the punishment mechanism be gamed? Can you describe an attack scenario against it?
If there's nothing at stake (in both senses) with staking, your only stake is that the system works well. If the random deterministic choice of who can stake, is random enough, then it is also essentially impossible to BRIBE you into staking, because in order to find out WHOM to bribe, you'd need to spend a lot of "proof of work" and chances are you'll find one necessary staker who doesn't accept the bribe.
I'm not sure if I understand your reasoning. Anyone could be bribed to do whatever action (i.e. only build upon certain blocks, censor some kind of transactions/blocks etc.) and without block rewards the bribees would lose nothing (except maybe suffering from a loss of value of their stake due to the attack as such)
As long as all tx in the blocks are the same it doesnt matter, if tx are plain invalid, it doesnt need the whole pow consencus for a node to detect it, same for spam, nodes just need to know they are on the same chain of valid tx, and only rules out the double spent, and they can already know if a tx is valid or not given a certain chain without requiring POW consensus.