First, governments consistently think of the economy as a static model. So you add up how many online sales you have, add in a percentage tax, and expect to make that money.
Second, Amazon pulling out or not is simply a short-term obstacle. The long term strategy is to put a stake in the ground, then continue to lobby and work at the national level. This sets precedent, even if it doesn't work.
Third, there's a lot of ignorance. You don't have to pass an economics test to become a legislator. I think sometimes in any group that deals with another group there's an "us versus them" mentality that takes over. In some folks' mind, if you're making money, they should get a share of it to spread around. Trying to adapt and configure your business model to optimize -- or simply just to survive -- is a sign of selfish, unethical behavior. Both sides of tax debates engage in a lot of emotional over the top rhetoric. It's easy to lose track of priorities and principles.