Most thinner walled pipe is actually very strong in most applications. A cross bar would be one. Think of the way the force is applied to the pipe. As long as the forces are not lateral or torsional, you're fine. If there is enough lateral or torsional force on that pipe to screw it up, the rest of the moped would be pretty much phukt anyway.
To give an unrelated yet similar example, when I install an oil furnace, I'll make the base plenum out of 24 ga sheet. Oil furnaces can weigh in excess of 200#. Add an AC coil to that, plus filtration system, and the rest of the duct, and you're putting a ridiculous amount of weight on a box made of such light metal.
As far as tube steel is concerned, think of the amount of abuse a child's swingset endures. It's no thicker than 22 gauge.