It's well-established law. The legal obligation is no doubt serious, but in a situation like the current market, where vacancy rates hover near 1%, the problem of locating a suitable assignee is not nearly so burdensome as it sounds. And the rule generally applicable to the situation is that the landlord "may refuse to allow an assignation or sublease," but "not unreasonably." This means the landlord only has a veto right--for good cause--over the tenant's choice. It doesn't mean the landlord gets _his_ choice. The reason for this law is because the vacating tenant will ultimately still bear some legal responsibility through the term of the lease, so he should have the right to determine with whom he contracts that term.
Tenants should also be aware landlords and their form leases in this city very often misstate or blatantly misrepresent their actual rights; it is the governing common and statutory law that controls each party's rights, _not_ whatever wording the landlord and his lawyer have decided to insert into the lease.