If you think of the Nexus card like your passport, then it's easier to understand. That is, they just get you into the country.

For example, if you entered the country for 4 months, left, then re-entered the country a couple of months later, using your passport, there'd be no problems. You'd still be granted leave to enter for a maximum of 6 months, but there's no way to tell when you enter if you staying for 3 days or 3 months.

No matter, you'd still be allowed in for the full 6 months. The problem is that you'd suddenly get involved with tax issues and the like. You will not have done anything illegal, just incurred extra responsibilities.