Thank you all for your responses. Although, I still don't see why this is allowed to be done. If manufactures can install 15amp plugs and draw 20 amps through them why have a 20 amp plug design?
You need to look at Article 630 for welders.
For that 20 amp, 20% duty cycle welder you would only be required to provide a conductor rated 9 amps.
Now obviously we would not drop to less than a 14 AWG but ...
Lets say your welder was 100 amps with a 20% duty cycle
Table 630.11(A) tells us we could multiple the 100 amps by .45
100 x .45= 45 amps, so we could run 8 AWG THHN and use a 100 amp breaker to supply it.
Why?
Because of the welders duty cycle, a welder with a 20% duty cycle can only weld for 20% of each hour which means the conductors will not be overheated.