If that was how the battery swaps work, then I'd agree, but that isn't what Nio is doing. You are paying a monthly fee for the battery in addition to paying for each swap.
By your own air quoting you know they don't own them.
The gas companies own those tanks and responsible for inspecting them whenever they rotate back for a refill.
The also choose the economic deal, they've baked a "lease fee" into their costings and are happy enough to ignore tanks that are never returned as most are returned, the total volume is high and the costs of chasing strays (that have an expiry date in any case) are greater than the value of gathering them back.
It's possible that EV batteries will one day have the same deal as propane tanks, more or less wrt to an explicit lease fee Vs an invisible one.