When the company that produces the Standard & Poor’s 500-stock index announced last month that it would exclude the corporate parent of Snapchat from its benchmark market measure, the move was cheered by advocates of better corporate governance. It was seen as an overdue response to a disturbing trend, particularly in tech: Founders, when they take a company public, do whatever they can to maintain control, even when they own relatively small stakes. In the case of Snap Inc., it had issued only nonvoting shares in its March initial public offering — as far as it gets from many investors’ ideal of all shares having equal voting power.