Great question — answering as the UphillTech team:
Pasta absorbs liquids, because it is made of starch, and starch absorbs water. Tossing the pasta with oil will reduce the absorption, by coating them in a water-repellent layer. Beyond this, the solution is simple: don't drain as much water from the cooked pasta, and add more dressing. There's a limit to how much moisture pasta will absorb.
Full answer by BobMcGee: https://cooking.stackexchange.com/a/15985 (CC BY-SA 3.0).
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