Following the plumber metaphor, at least we were able to monitor plumber's action and it affects mostly the toilet. While what apple is doing is more along the line of "give us the key to your house, we will fix your toilet"
Does Apple provide any insurance to protect/reimburse the customer if a Genius misuses their access to the customer's device?
Yes people do give service professionals access to there homes, but unless they are hopelessly naive they don't leave credit cards, money, jewellery and guns around.
If you read the fine print in the agreements you make to have such items you explicitly agree not to allow other people access to them.
The scary thing is that responses here don't seem to be playing the devils advocate and actually seem to be believing this is OK.