Near as I can tell, it's an afront to their ability to read the Bible in plain English and understand it perfectly. If you must take into consideration the culture, language and historical understanding; not to mention the logic of which Joses is which and which Mary is which, and why is the Mother of Jesus called "the other Mary" you begin to question your ability to know all by yourself.
It's easier to simply take the words at face value, pluck out quotes as needed and use them as a cudgel to beat up on Catholics.
SD
I think it's somewhat ironic that someone can read a few Jack Chick comic books and somehow become "inspired" by the Holy Spirit so that they themselves can interpret scripture -- all while claiming that theologians for the past 2000 years have interpreted scripture incorrectly.
If Catholic Marian beliefs are so offensive, why didn't Luther or Calvin denounce them (they in fact agreed with them).