Truck drivers are often owner operators on exploitative leasing terms or training terms with their employer that dictate they must work X amount of time or they owe Y training cost, usually in the 10s of thousands of dollars. In addition to this, drivers are typically paid by the mile, which means they’re not paid at all for any time they take breaks, or even while trucks are loading or unloading. Not to mention the razor tight schedule they’re expected to make. Piss bottles, like every other uniquely horrible American cultural artifact, are a result of overworked and underpaid workers trying to make the best of their situation. They’re literally saving time and making money by pudding in a jug.

Sometimes it's just an inconvenience. This gas station is located in a rural town in the foothills of Tennessee which is at the convergence of two major highways. This appears to be the only truck stop within 20-30 min in any direction. The drivers could pull into a normal gas station or just pull off the road, but the roads and parking lots in this area aren't really designed for vehicles of this size.

Some aspect of it is certainly over demanding bosses and workaholic drivers, but another part of it is just not wanting to bother with finding a place to pull over without going out of the way. Also, it never seems to be as simple as just pulling over to use the restroom. You have to pull off the highway, go down the ramp, if you're lucky the gas station is right there at the exit, but if not you might have a short couple min driving. By the time you've gotten off the road, found a place to go, got your truck parked in a normal parking lot, and went in to actually use the restroom a lot of time passes. Probably 15-30 min per stop.

Their bosses put them under pressure to just not ever stop.

It's the way of the road, Bubs