In a permissioned/private context, the validators have an additional layer of trust. Namely, that they have access to the blockchain in the first place. Bank A is unlikely to 51% attack Bank B, when everyone who has access (read: Bank B's lawyers) can see what they did. Ergo, there's little reason to additionally incentivize good behavior/disincentivize bad behavior. Similarly, the economics of new blocks (inflation? no inflation but mandatory fees?) are generally irrelevant, because validators have their own reasons.
In a public blockchain, there's no such trust. If validator 0x45cdf112... attacks the network, there's a good chance that no legal trail to its masters can be found. The best that the protocol can do is threaten and bribe validators into behaving, which immediately opens a whole field of game theoretic questions. Additionally, since validators may be validators solely for profit, economic questions become important.