It's because lactose-free milk (or at least the Lactaid brand that I see my roommate drink) is treated at higher temperatures. You can find regular milk that has gone through the same treatment labeled as UHT or ultra-pasteurized, like the Horizon Organic brand and another I've seen at Costco as shelf stable juicebox form factor milk drinks (UHT coincidentally removes the refrigeration requirement too if stored in an appropriate container).
No need to pay extra for the lactose-free variant if you don't really need it and the plain UHT-treated milk is cheaper :)
There was this Mythbusters episode (pyramids preserving foods) where they sliced an apple in half. The one half rotted fairly quickly at room temperature the other was fine.
Brief investigation showed that there was difference in the initial contamination of the two halves and the results observed.
Modern kitchen and fridge are fairly clean places unless you are a movie bachelor. So opening a milk bottle and pouring a glass won't contaminate too much. And at 4 degrees there is not much breeding activity in bacteria anyway.
Actually, they didn't appear to investigate the cause at all - they just assumed one cut surface must have been contaminated more than the other and changed the experiment to remove the cut surface altogether. Since the mold on the non-pyramid apple only survived on the cut surface, not cutting the apples made sure no mold grew on any of them, and all bar one of the apples (the one from the cube) was perfectly preserved at the end of the experiment.
Was kind of annoying actually; the Mythbusters got so pissed off that their experiment appeared to show pyramids working that they set up their next test in such a way it'd give the same result regardless of whether pyramids or contamination had caused one apple to rot faster before, then held it up as proof pyramids were bunk. While it's really unlikely pyramids can magically preserve food, they should still have been honest and fair in their testing.
Regular milk spoils much faster after you open it though. Everything that I can find on the web suggests that UHT milk expires just as fast as regular milk after opening.
No need to pay extra for the lactose-free variant if you don't really need it and the plain UHT-treated milk is cheaper :)