Because it takes one of the few remaining inherent strengths of the park over the competition that SEAS can't fully eliminate without demolishing the park (themed areas, where visually and through sound you can be "immersed" in a foreign country inspired land) and just deletes a huge chunk of the immersion factor. Same reason why I personally don't like when they change the entire park sound track for things like Mardi Gras and whatnot, though the double dose of shitty pop music being both against the theming and making it feel like a SF park is probably why people despise it significantly more than the other soundtrack changes.
As with most reasons people get upset about something BGW related, it just hurts the feelings man. People have an extreme emotional connection to this park, I'd say an above average size of people relative to attendance compared to most regional theme parks, because for a while this place was special and unique. A lot of the times people are upset not because of the logic you can put on paper, but because it just sucks to see where we are. This addresses more than just your question here, just a general statement for all things "why do hardcore BGW fans care so much about this one stupid thing". Hell I still hate when I am in England and Brit rock comes on instead of classical music.