It seems they already are illegal, at least in some jurisdictions.
> "In June, for instance, Washington state Attorney General Bob Ferguson, who is also litigating against the merger, fined Albertsons $25,000 for imposing a land use restriction on a store it sold in 2018 in a low-income section of Bellingham, Washington. As part of the sale, the supermarket giant put a requirement on the deed that no grocery store could open there until 2038. Ferguson found this provision was a violation of the state antitrust law."
The fine imposed doesn't seem all that significant though.
Perhaps they could also be prosecuted under federal antitrust law?
That depends on which court ruled against Albertsons in the above case. Higher courts are under no obligation to honor a lower court’s ruling; they can and do overturn them.
> "In June, for instance, Washington state Attorney General Bob Ferguson, who is also litigating against the merger, fined Albertsons $25,000 for imposing a land use restriction on a store it sold in 2018 in a low-income section of Bellingham, Washington. As part of the sale, the supermarket giant put a requirement on the deed that no grocery store could open there until 2038. Ferguson found this provision was a violation of the state antitrust law."
The fine imposed doesn't seem all that significant though.
Perhaps they could also be prosecuted under federal antitrust law?