Well, but then what would be the consequence of the customer doing work? I mean, either there is some legal consequence to it, then it's a bad idea, or there is

Well, but then what would be the consequence of the customer doing work? I mean, either there is some legal consequence to it, then it's a bad idea, or there isn't, then it's useless?! Especially considering that the mere indication that the appliance was opened does not tell you anything about what was done to it and thus whether there is any likely connection to a given defect. And while you might not need to repair appliances under warranty, mind you that there are other reasons for opening appliances, from curiosity to quality control to wanting to modify things for your needs. Whether these companies want me to do repairs really seems like a completely irrelevant interest. It's my property, and if property means anything, then their interest to protect their trademark certainly does not trump my interest in having control over my property, or else you could use the same reasoning to justify pretty much any right for them to control and limit what you can do with your property.