$250 to replace a bum plug in an in-warranty motor? Odd.
Where did you buy the motor? Did you get the filled-out PDI (pre-delivery inspection) form? Did you contact your dealer? Has your dealer let you down, or is this a run-of-the-mill warranty issue?
All motors of all brands can and do have occasional warranty issues. The outboard brand is unrelated to the spark plug problem -- rare as it is. Most Japanese motors run NGK plugs, and I have had more issues with the plugs in Yamahas than Tohatsu motors. Given that Tohatsu, Nissan, Mercury and Evinrude all come out of the Tohatsu plant (with different decals), making Tohatsu the largest (motor for motor) manufacturer of all outboards, blaming the manufacturer for your issue seems inappropriate.
Why would you pay anything for an in-warranty repair? At under 5 years, your motor should be under warranty.