I think it is because he is treating all muslims as terrorists. I dont personaly care but i have some friends that are muslims and they feel so bad after that ad and society treats them differently. Trump is really impolite, is he wants to be the president he needs to work on that.
Not just Trump's, but many political ads go too far. Trump is especially guilty of hate and fear mongering, and his ads show locations that don't match up with his words e.g. Morocco instead of Texas. He's not concerned with facts, just hype and fear. The New Yorker referred to it correctly as "Trumps Ugly Ad".
Trump's first TV ad is yet another example of him crossing the line and taking things too far! First off the images he uses are from the Moroccan border, not the Mexican border, and he is attacking on the fear of ignorant and uninformed citizens of the US and offending too many people.
As far as political campaigning goes, nothing is too far in my opinion. The whole point of a campaign and advertisement is that people talk about it, whether it is positive or negative context. Donald Trump used a tactic of shocking the viewers and making them think about the ad long time after the viewer has seen it.