Let's just admit, for the sake of argument, that the UAE will not control any security operations.
What will they be paid to do? At minimum, the actual physical movement of shipping containers.
Completely US-controlled security watches that movement. Every last container and its contents. Perfectly. Can you say "easter bunny"? How about "heroin"? Or "ecstasy"?
So, let's forget about trusting our own intelligence/security agencies, the ones who kept 9/11 from happening, you know, to protect us perfectly.
The risk is in the fact that those actually moving, filling, emptying shipping containers will potentially be bossed by UAE end-to-end.
You say that when UAE buys the British company, all the employees will remain British? I seriously doubt that. At least the very top tier will be replaced with loyal UAE subjects. And those are the ones who will say how operations go.
That makes our line of defense the US citizens who you say would not sell out their country.
We don't have to look too far to find people who would sell out their fellow citizens. Why the hell do many of us have guns? To protect ourselves against exactly such people who are happy to kill you and your family for your wallet.
Charles Manson. Jeffrey Dahmer. Gacy. Son of Sam. All US citizens.
But none of them attacked the US itself, only its citizens.
How about Tim McVeigh?
Looks like that line of defense is already down.
Giving a questionably friendly country end-to-end control of the aspect of port operations that could damage us (that aspect being the actual shipping of stuff) is a stupid idea.
So, why not be unhappy about allowing the Japanese to own buildings here? So far, they haven't attacked us on our own soil (one of the states).
You might argue that the chances that everything can be lined up perfectly, like finding enough disgruntled US longshoremen, paying somebody to forge container seals, having security people munching donuts rather than inspecting, and all the things that would have to happen to get a nuclear weapon shipped here are very very small.
But since the explosion would be very very big, I don't want to support something that would make the chances any bigger than they have to be. Let alone support something that practically begs for it.