Remember, the ancients were fully aware that it wasn't the deity—it was just a manifestation of the deity. But at the same time, it was the deity. Confusing? Maybe. But because in a very real sense it was the deity, Isaiah and the other prophets could have a good time making fun of the whole process.
At the risk of incurring the wrath of the few people who actually read this blog, I could say we can compare it to the treatment that is given to the U.S. flag. It isn't the country, but it represents in a very real way the U.S. That's why people get so upset when people burn it out of protest. They are symbolically burning the country. Or, why the flag is never supposed to touch the ground, or you stand and salute the flag, or say the Pledge of Allegiance, or any one of a number of other "silly" rules about handling the flag.
Side question? Is the flag an idol?
You figure it out, but I would suggest it is...just as nationalism is an idol. Yes, especially "American exceptionalism."