Union solidarity is a myth.
David Freddoso has the details: essentially, the Teamsters and the UFCW are at cross-purposes when it comes to the Giant supermarket chain here in DC. The Teamsters are angry at Giant because Giant parent company Royal Ahold is closing a Teamsters-controlled facility in Jessup, Maryland, in favor of a non-union one in Pennsylvania*. This will be happening, and the real question is whether Royal Ahold will start the process with another Teamster-controlled Jessup facility. As Freddoso mentioned, part of the Teamster's push-back on this involves directly attacking the Giant supermarket chain's reputation... which if successful will of course adversely affect the UFCW's own workers.
Freddoso couched the whole thing in terms of Wal-Mart (which - like Target - is aggressively getting into the groceries game), but it's really more a question of relative power structures. Frankly? The UFCW is a joke; at least, it was a joke back in the 1990s when I was an (involuntary) member of one of its local supermarket unions, and it's highly unlikely to have gotten better since. It collected union dues, and that's about it. In contrast, the Teamsters are, well, the Teamsters - which means that it's a powerful union that can negotiate fifty grand lump-sum settlements for its members.
Match that up against minimum-wage cashiers who often can't get full forty-hour shifts (a practice that often neatly avoids agreements about minimum pay and benefit requirements for full-time employees, and one that usually has the unspoken approval of union leadership), and it's no contest. The Teamsters' first rule is to get theirs... even if it means that everybody else gets theirs, too.
Moe Lane (crosspost)
*Which is in the process of having its labor union reform issues being addressed: in marked contrast to, say, Maryland.