Because their 'wish' is an attack on the freedom of religion of non muslims. Not drawing their prophet is a religious rule, just like eating kosher or sundays for rest, it's for the people of that religion to observe (or not). If you claim no one can draw the prophet you apply a religious rule to non religious people, and that's crossing the line of freedom of religion into theocratic territory.
So in essence this is a claim to islamic rule over the public sphere, and not the only example. This is not about hurt religious feelings or muslims beeing disturbed in the practice of their religion, this is about muslims being hurt in their desire/demond to rule over non muslims. If no one stands up for freedom of/from religion, it's gone. What muslims tend not to understand is that freedom of religion works both ways. Their right to practice their religion in the West is the same right as the right of other people to ignore all religious rules.
And let's not fool ourselves, the apologies and general giving in to muslims who claim to be offended is not founded on thorough insights on the multiracial society but about taking the easy way out. Avoid controversy, protests, accusations of 'islamophobia' and beheadings. Cowardice would cover most of it.