I am with Leo on this. That is a big mistake using a low nap roller. What ends up happening is that you do not put enoght paint on so that material can not flow out.
Whenever I paint walls, ceilings or such I always use at least a 1/2" nap wool roller cover. That way you get plenty of material on and the wool roller cover cleans real easy. They are so much easier to clean than those cheap covers.
Ditto what Leo said. I just finished painting several steel doors with latex and a 3" wool roller. First coat you could see through in places and the roller marks were quite obvious. Once that coat was dry to touch, a second coat gave good coverage, and a third coat did the job.
Combination of nanny state and enviro watchdogs. If they allowed more than a quart someone might sniff the fumes to get high and cook a bunch of brain cells (that they could do the same with a quart is immaterial). The thinner/carrier for oil based products are VOCs so they want to limit their availability. The fact that you can probably buy mineral spirits or other thinners by the gallon is also immaterial, apparently.