SINGAPORE, Jan 29 (Reuters) - The chief executive of brokerage app Robinhood said he made a “difficult decision” to restrict buying in more than a dozen volatile stocks, including GameStop and AMC Entertainment, to protect the company and its customers. “We absolutely did not do this at the direction of any market maker or hedge fund...the reason we did it because Robinhood as a brokerage firm, we have lots of financial requirements,” Robinhood’s Vladimir Tenev said on CNBC. “In order to protect the firm and protect our customers, we had to limit buying in these stocks.”