Sometimes people don't think too far past the end of their noses. I mean they don't pronounce bowling like howling in the U.S. so it shouldn't be much of a stretch to pronounce Rowling like bowling instead of howling. sheesh.
Actually this one interests me a bit. Not that I care about JK's name (and BTW the end of the last HP novel sucked big time), but the way that the pronunciation/spelling of words are changed by "the media" to suit their audience.
One of the biggest examples of this is the terrorist group formally known as Al Qa'ida. If you listen to news reports from 10 years ago the name was given as Qa'ida and pronounced with 3 syllables, but over time it slowly morphed into Qaeda*, with only 2 syllables. I don't know if this is because the news media thinks their audience can't handle funny sounding words, or if there is some conspiracy to subtly change the name as a way of giving the middle finger to the members of the group.
* I'm going to go out on a limb and say that I think that the pronunciation change is more noticeable in US media.