so the the question becomes 'why municiple bonds are not taxed?'
Munis are a timebomb, imho.
Adding zeros is not that hard. Munis have actual tax bases and actual obligations and those seem out of step in some situations. That muni debt will be mispriced because of tax incentives seems obvious.
Municipalities are in the same spot that Greece or Italy is in. They have debt in a currency they do not issue. They cannot devalue or inflate that debt away. Therefore they have to manage their debt as a corporation or state would, but they are much smaller and less flexible. Bad buy.
Think of it as a way to take money from state and federal taxes and to move them to municipalities etc. It's either that or the government above would have to pick up the slack.