Arbitrary rules are clear, yet, but also blunt. If you were building a capital intensive product (see most of the alternative energy/biotech space - to get a prototype built costs millions and millions of dollars in most cases), 250k investment is really low, and probably would have no bearing on the success of a company.
But for a small tech/software company whose only expenses are its 2-3 founders' accommodation and sustenance, 250k is a lot. So maybe some arbitrary rules are the right thing, but there needs to be some common sense and differentiation across different sectors. Not all startups are the same.
The current E2 visas don't include a hard-and-fast minimum investment. It has to be sufficient, though : and the interviewer in the Embassy will be the judge of that, ultimately.
For an Internet technology startup, $100k is enough (proven in 1999, and renewed ever since).
