Section 6.5 purports to give a game-theoretic justification for why a node should include as many valid signatures from its peers when proposing a block, for fear of retaliation. But nothing stops me from dividing my voting share among, say, a thousand nodes which appear to be unrelated.
If one of those nodes decides to "defect" (steal a larger-than-intended share of fees by excluding some signatures) then the expected time before it is selected again can be made arbitrarily large, by making the node's voting share arbitrarily small. If that expected time is significantly larger than the unbonding period, then there's no way to retaliate, is there? It can just unbond and then transfer its coins to a "new" node.
EDIT: I originally typo'd "arbitrarily large" instead of "arbitrarily small" above, but hopefully the meaning was clear.