Why am I being downvoted for stating my reasonable opinion?

In a straightforward disagreement about which interpretation is right, it's also reasonable to mildly downvote the one you think is wrong.

Ah. That's a shame... as there is no button or indicator for "mild".

Making the behavior for "I disagree" and "this is erroneous" the same seems like a problematic design.

Downvotes shouldn't be used for disagreement.