All this means is that they were searching trash bins outside his apartment, so, unlike the earlier searches, his consent wouldn't have been sufficient to authorize the search. (They were searching through a mix of garbage, including other people's. Hatfill's consent would be irrelevant to a search which included other people's trash; therefore, they needed a search warrant.)
I thought it was perfectly legal to look thru a suspect's trash, as long as no trespass is required to gain access to it.
I seem to recall a dispute a while back between Microsoft and Oracle in which Oracle allegedly decided to look in the trash of an organization that was lobbying for Microsoft. Oracle's rentacops supposedly rented space in the same office building, so that they would not need to commit a trespass in order to get at the dumpster.