The same can be said for a lot of software developers. How many people spend their days gluing together existing libraries vs. writing their own? This rule seems ridiculous; I fail to understand why they're are different tax write off rates for employees based on what they're doing. Either way they're being paid by the business!
The entirety of the rule seems to exist to punish small players in the market. A barrier to entry to box out competition.
The difference is repetition. Many welders work some form of assembly line, where they constantly are welding the same bracket on and sending the part down the line.
Even if they are not on a line, few welders are designing the bracket, instead they cut it out according to blueprints (this might be a separate person) and then weld it on. Then they look at the next part of the blueprint and put it on.
The entirety of the rule seems to exist to punish small players in the market. A barrier to entry to box out competition.