I had noticed that as well. I wonder if it's because the lactose contributes to the process that makes it go bad, or because they treat it to make it last longer assuming it will move slower with a limited consumer base.
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Ah, a little googling, and it turns out it's a byproduct of the procedure to remove the lactose, Ultra Pasteurization (UP) or Ultra High Temperature (UHT) pasteurization. [1]
1: http://chemistry.stackexchange.com/questions/34884/why-does-...
I can confirm that lactose free milk never seems to go bad. And I don't know what the situation is in other cities, but in Seattle, it's ubiquitous, and it's not even specialized brands (e.g. Lactaid) any more.
Yeah here in the Bay Area there are multiple store brands that have lactose free, and they are almost all cheaper than Lactaid (although Lactaid seems to go on sale here a lot bringing it in line with the store brands).
We buy 1.5 gallons at Costco for the two of us and never had it go bad.