That's no excuse, they can make the cans bigger, like 140oz if they want extra room. Extra room isn't needed, except to reduce blowout caused by dropping or other impact that could cause hydroshock.
1.3 gallon plastic Aspen cans do have the seal ring on their caps.
I tend to fill my cans as full as possible, and they don't swell in the heat since there's so little room for vapor to build up.
Venting is hazardous because it could cause fire/explosion, spills, and breathing health hazard. Just look at how modern EPA gas cans try to capture and seal in vapor with the result of cans swelling or collapsing with temperature change.