Merchants just want more sales. If BTC can bring more business, it's good. Besides, I guess it's quite fashionable too, and it doesn't cost much to implement, so let's do it.
I would have to disagree. The companies mentioned above that began to take Bitcoin as payment probably had no interest in Bitcoin themselves nor did they particularly care about increasing their sales through the currency. Many of their customers I'm sure do not use Bitcoin as a payment option because paying in fiat is just as easy as paying in Bitcoin to major, secure, retailers. It probably did end up costing them some money to cut a deal with Bitpay or whoever their 3rd party exchange was. They may take a percentage of each sale, or something kind of fee. The reason I see many companies taking Bitcoin is for fraud. With Bitcoin, a company can't be held liable for fraud or chargebacks as often paying with fiat. That, or maybe it was a domino effect after Overstock.com started to accept Bitcoin.
For fraud? Are you serious? Do you really believe companies like Microsoft or Dell are trying to hide a few sales? If they do, they would be hugely stupid.