Edit for spelling errors
There is a non-trivial risk associated with repairing appliances connected to the power mains - incl. electro-shock of the repair man... or the users.
Water damage, leaking, flooding for washing machines, dish washers and the like.
On a side note my son accidentally sat on his ereader, a popular model, but there is no place to buy a screen. There were a couple on eBay but they wanted 20$ less then a new ereader. It is frustrating.
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.
Even simple maintenance like dust cleaning would "void" the stickers.