You raise a good point, although there might be some difficulty with legislation requiring people to declare their income to investment vehicles.
The basic problem is that while something like Diaspora can attract goodwill funding by being in the right place at the right time - I kicked them $25 in exchange for a t-shirt and the vague hope that they can advance the state of distributed network services. But most people who want to invest in something are looking for an ownership stake, and as soon as you offer them that you've initiated a much more complex contractual relationship and are subject to a very different legal regime.
Rather than an income percentage cap, there might be some mileage in a small monetary amount. For example, commercial transactions of goods above and below $500 are treated differently. There are already provisions in Regulation D for companies that don't need to raise more than $1m or $5m in a single year, which might be great for startups, but those companies appear to have less of a legal 'safe harbor' than those which can raise an unlimited amount.
The 'safe harbor' provision is in SEC regulation D, rule 506: http://www.law.uc.edu/CCL/33ActRls/rule506.html in subclause (a), it's the exemption from rule 4(2) of the Securities Act of 1933 which gives firms the legal security blanket. Although rules 504 and 505 limit issuers to smaller amounts (but still quite sufficient for many new businesses), they don't have that specific exemption, and so the founders are implicitly subject to the prohibitions on selling or offering shares across state lines.