Ruflyf,
Completely normal...I've heard two reasons for the A/C being tied in to the defroster.
1. A/C air has less humidity, making it "drier", so it's more effecient removing moisture from your windshield.

2. The A/C pump turns every time you use defrost to keep the oil in the pump circulating, keeping the seals moist/lubricated during the winter.

Ldub