It may be in breach of their contracts, in which case their employer is within their rights to discipline them or sack them, but I don't see why it's 'greedy' or indeed morally reprehensible if they are fulfilling all their obligations in terms of the distance learning the school wants them to provide.
Even if they need to be available online to help pupils during their normal lesson times, there is in theory no practical reason why they couldn't do tutoring during free periods if that were allowed by their contract.
so how come they can't do zoom lessons for their state pupils?
Because that is their school's policy, not because individual teachers have decided they can't.
Gosh, doesn't quite fit with the "we're on our knees!" narrative, does it?
Gosh. It's not a narrative. Some teachers are on their knees, many are not. Unsurprisingly, it depends on their school's distance learning policy and indeed the individual teacher's contracted working hours. Some schools have not been doing much. That is not the decision of individual teachers.