Here is a scenario that can happen. They spend the 2-3 hours installing it, and find out there is a problem with the part (just because it has a Honda name on it does not mean it can't fail). So now they have to pull it out and spend another 1 hour doing so. Say you take the part back and warranty it yourself, after you get a replacement, they spend another 1-2 hours installing. So will you be willing to pay the 5-6 hours of total labor? Lets say if it works from the get-go, then 6 months later, it fails, who is going to pay for the diagnostic, removal, repair. They don't want to be in the position to argue with you if something bad happens.