They *dare* because of the customers. People who
would never dream of
working on a TV set will eagerly open a PC and cause all kinds of havoc. I
use warranty labels in my shop, and the warranty is VOID if the seal is
broken, but if I know the customer has a reasonable level of skill,
tempered with caution, I tell them not to worry about breaking the seal.
PCs, unlike televisions, are sold as being user-upgradable--it is no more
"servicing" to add a card or drive to a PC than adding a component to a
high-end stereo is.
I would probably be frustrated myself if customers tried to return items
they destroyed. It's certainly your prerogative to sell PCs with
warranty stickers. To me, though, that's a deal-killer, absent the kind
of relationship you appear to have with your knowledgeable customers.
--Mike