It seems to me that almost every summer I go through a tire. We drive a lot. I always carry a tire plug kit and a 12v pump. Comes in handy when you are way out in the backcountry exploring.
I always figure the plug is good enough to get me to a tire shop or worse case back home. I always have a spare tire too but I'd rather drive on a plugged tire than a donut (compact spare).
I would guess the tire shop in question was more concerned about the liability of repairing a tire than anything else. From their perspective it is likely not worth the hassle to get a few bucks for a repair.
Myself, I never skimp on tires
. They are what keeps you (and your trailer) on the road. Buy good ones, replace them when they are damaged. Don't cheap out when safety is on the line.