You're right to look at the marginal tax rate but you're not considering all the options available. For goods like a hamburger it's easy to see why a tax on hamburgers makes people buy less hamburgers. Hamburgers cost more with the tax, so you can save money by buying chicken nuggets instead. This doesn't mean everyone will switch to chicken nuggets, just that many people who slightly preferred hamburgers to chicken nuggets before the tax will now switch over and slightly prefer the chicken nuggets after the tax.

For business and entrepreneurship the argument is the same. Increased taxes and other types of costs decrease the rewards so you'll end up with less of it. It's not that business owners don't want more money. It's that they are continually deciding between many options.

For example, years ago I had to decide between going to graduate school, getting a job in industry, and starting a business. The tax and subsidy breakdown is drastically different between all three. Of course tax implications wasn't the biggest factor in my decision, but it mattered.