To be honest you are assuming that they are not keeping a % of the payment in BTC with their payment processor.
Some merchants may do that (just as some merchants actuall accept bitcoin directly, without BitPay's intermediation). But I am pretty sure that Dell, Microsoft, Wikipedia, etc. do not. Handling bitcoins requires extra work from accoutants, which is not worth the amount of payment received. Losses from bitcoin price variations (as Overstock and Fortress suffered) would have to be reported in the quarterly reports, and would be hard to justify to stockholders.
Also what is important to note is that BTC like USD,CAD,YEN,CHY,GBP and any other currency is being accepted for payment

SIgh. No, it is not the same thing. Dell USA
does accept USD, because you can wire USD to their bank account directly. They
do not accept CAD,YEN,CHY,GBP
nor BTC, and you
cannot send them any of those currencies directly; you must send them to some
other company, that converts them to USD and sends the USD to Dell.
Jorge, can't you see that we are arguing about what 'accepting' means? Everybody agrees about the model, the underlying chain of events that takes place during and after a payment (if a company chooses to get paid fully in fiat). It's just that when a company allows you to pay for products in BTC, I call that "accepting BTC", whereas for you "accepting" apparently means whatever you are getting paid by the payment processor at the end of the process.