For arguments sake, lets consider an average potential hire. Lots of startups are willing to pay ballpark $130k in San Francisco right now (low end $70k, upper end $180k). Since the company most likely don't have funding lasting more than 18 months, this means they're budgeting just about $200k plus taxes and benefits for this person over that duration.
You might be willing to pay 10, 20 or even 30% as a fee for the right candidate (just like headhunter fees) but that translates into either that many fewer employees or that much less runway. Taking 18 months of capital and turning it into 12 is a quick way to end up dead. By contrast, large companies are paying much more in total compensation and can easily afford to spread this cost over multiple years (assuming the candidate doesn't nearly immediately utilize his ability to transfer his visa).
If anything this proposal seems like it would lend itself towards ensuring only large companies apply for H1-Bs and that smaller ones just have to hope people will transfer, despite not being able to promise that they'll be in business long enough to sponsor a green card.