If they repair or create objects and do not get paid more than "break-even amounts", that would be a hobby. One would also expect them to enjoy the activity to label it "hobby". Many homeowners enjoy doing home improvement projects themselves, but would scoff at the idea of full time involvement in that type of work. Same thing here,only welding is being done. I guess it boils down to living in a free country where one can do anything which doesn't violate the rights others. Everyone can't do needlepoint and ships-in-bottles you know!
tpob
I am employed in the aircraft industry as a design/manufacturing engineer and as a "hobby" I (and my sons) race karts (go-karts to use the US terminology). I build, modify and repair the kart chassis so I use a variety of welding techniques (Mig, Tig welding, Tig brazing, o/a welding and o/a brazing. I am not a "professional" welder (ie I have no qualifications and I do not weld as part of my day job) and, although I consider myself reasonably proficient, I feel that I can only call myself a "hobby" welder. Also, I cannot justify the purchase of expensive "professional" equipment so I usually buy the "hobby" welders (ie the equipment that maybe does not have the duty cycle that the professionals would require and/or equipment that is generally of lower power than the guys who make a living out of welding would typically use).
I hope this answers your query
IanT