So, when an account leases forging power to a node, what should prevent that node to mis-use that power?
Fear that the leaser will take the power back.
That should not suffice. There could be a business model made on top of this:
1) bad node tells forgers 'u get a bigger slice of the profit when u support me'
2) bad node does only accept transactions with big fees
3) people will pay
You could argue that in that case people will cash out and move on to another crypto-currency or NXT clone. That is true if NXT is young and nobody depends on it as it is the case right now.
However, if NXT is mature and a lot of services depend on it, nobody can cash out that easily and will feed the scheme
Will work only if a lot of power is owned by bad nodes. This is unlikely if Nxt is mature.