Tax Incidence Questions

1. Why did Amazon get very upset that they will have to charge customers sales tax in California even though only Amazon's customers pay sales taxes? 
2. Should you care if your employer pays half of your payroll tax or if you pay all of it? 
3. Renters tend to be relatively poor on average and landlords are richer than average.  Are real estate taxes progressive because relatively rich landlords (and homeowners) pay them rather than relatively poor renters?  
4. Are property taxes on the value of land or on the value of buildings going to have a different impact on people?