The law was different back when Google was having its IPO.
The Securities Exchange Act of 1934 sets forth certain requirements for companies to register their shares with the S.E.C.
Specifically, Section 12(g) requires that a company register its securities with the S.E.C. if it “has total assets exceeding $1,000,000 and a class of equity security … held of record by five hundred or more … persons…”
Sounds to me like this law has been around for a while.