I'm new to C&C, but the modules I've read so far (Beneath the Dome, Hel Rising) seem to give all of the monsters maximum hit points. What gives? Is this a C&C thing?

You can determine HP however you wish. As for why they defaulted to max, I have no idea, but is not due to some official rule some where.

No, it is not a "C&C thing." It's a design decision by the author. Perhaps to ensure that XP calculations are easier. Perhaps to make the monsters "more difficult."

If you know that that's what's happening and it's consistent, it's kind of nice to show max hp; then you can make up just about any number that's less than that, or always shoot for a little over half, and not be too unfair.
Of course, you are free in any case to make something up.