If I visit a site on my phone or tablet intending to buy something and the site doesn't work for me, I will go to another online shop whose site does work on my chosen device. As a customer, all I know is that Joe's Widget Store works on mobile Safari whereas Jack's Widget Store does not.
Developers, suits and designers should meet to their customers needs and wants (obviously within reason) because the customers are the ones who have an interest in buying your product. There was a recent news article (Black Friday I think) which showed that iPhone/iPad customers spent the most money via mobile shopping. Do you really want to alienate such a large potential customer base because you think it isn't your fault?
(Note: I would argue that failing to test your site in the most common web browsers, and modify the code to correct any weird stuff, probably is your fault).