I believe Pound changed the punctuation purposely. It was part of the editing process. The poem was originally published with gaps between the words, stressing the rhthym. This was unnecessary -- the rhthym of the poem is clear. He changed the colon to a semicolon because the colon implies "are like." The ideogram is works a bit differently than metaphor/simile and so he substituted the semicolon. I read an article in a journal a few years ago regaurding this topic but I've forgotten which one.