The marijuana regulation law that Colorado Gov. John Hickenlooper signed last month includes a quarter-ounce limit on pot purchases by visitors from other states. Colorado residents, by contrast, may buy up to an ounce at a time. As I have mentioned before, making the purchase limit hinge on residency seems inconsistent with Amendment 64, the marijuana legalization initiative that is now part of Colorado’s constitution. The quarter-ounce rule may also be vulnerable to challenge under the U.S. Constitution, since it discriminates against residents of other states.