THey may, but the announcement is that they are going through with the deal. I don't think the U.S. can actually BLOCK the merger, at "best" we can terminate the leases, which means we have to pay them back the money and probably penalties.
Then we would re-bid the leases, and they could simply bid on them again, and there is nothing to keep them from WINNING the bids. If we instead restrict the bidding, the states and localities will have lost a bundle of money and have to raise taxes or cut services -- then lets see how important THEIR citizens think it was to stop this deal.
You are making a lot of assumptions. In my business I have signed many leases that are assignable to third parties subject to my approval. I can block assignment of those leases to another company without incurring any liability. There are companies that I refuse to do business with and I am not going to be forced into business with them because a contractor decides they want to sell my lease. The original company that signed my lease is still on the hook for honoring their contract with me and if they want to get out of it they have to find a buyer that meets my approval.
I am not sure what the situation is with the P&O port management leases but it is not automatic that we will owe them anything if we refuse to approve the transfer of these leases.
What would happen if they wanted to transfer them to Osama bin Ladin Port Management? Would we have to buy their leases back if we didn't like the idea of them transferring them to a terrorist? I doubt it.