I think the bigger reason is the fact that out of ignorance many Protestants think that the Reformation had something to do with hatred of Catholicism rather than widespread corruption at the highest levels of the Church. I have done extensive research about the Vatican in the 15th century and it is amazing the level of hypocrisy and greed; I wrote a very in-depth paper on the Borgia family (Rodrigo Borgia was Pope Alexander VI and father of Cesar and Lucrezia Borgia; if you want to be truly appalled, do a search of the term "Banquet of Chestnuts") when I was in college and they were truly despicable people. There is no point in denying that there was corruption in the Church at that time; however, the Counter-Reformation corrected these problems. Popes no longer have mistresses, children and armies, they no longer sell indulgences in order to build palaces for themselves and their families. However, it is also important to recognize that but for these men's lavishness, the world would not have much of the great Renaissance artwork that we have today.
But back to my main point, I believe that these bigots simply believe that hatred of Catholics is somehow critical to their religious beliefs.