The laws are anything but straightforward. For starters there are 50 States with each one having different laws.
Take Florida where I have personally represented employers and employees/Independent contractors in these types of matters. The FL case law establishes, what in my opinion is the very worst kind of law, a factors test. So essentially there are a number of factors the courts put forward, I believe 7 factors by my last count, and in each instance a court would have to analyze each case on its own merits, and by case I am referring to each employee/Independent Contractor not each company as a whole.
One of the most heavily weighted factors is who the company pays...does the company pay the employee/independent contractor individually or pay a company formed by the individual. It would be evidence enough to simply determine which start-ups have a policy against paying any individual and requiring all individuals form companies. If start-up does't require their Independent Contractors to set up separate entities the start-ups deserve what is coming to them, a finding by the Courts these were employees.