(adds T-Mobile comments)
By Alina Selyukh and Marina Lopes
WASHINGTON Aug 8 (Reuters) - The top U.S. communications regulator on Friday said he is asking all large U.S. wireless carriers to explain how they decide when to slow download speeds for some customers, after questioning Verizon Wireless about such a plan.
Verizon, the biggest U.S. carrier, said last month that the top 5 percent of high-speed data users on its older unlimited data plans might experience slower speeds starting in October.
In a letter to Verizon, Federal Communications Commission Chairman Tom Wheeler said he was "deeply troubled" by the plan and expressed concern the decision to slow data was based on consumers' plans instead of network needs.
Verizon defended the practice as a narrow, lawful and "widely accepted" way to manage networks and said all its competitors also relied on throttling of top data users to optimize congested networks.
But Wheeler on Friday indicated he was not convinced.
"'All the kids do it' was never something that worked for me when I was growing up," he told reporters.
"My concern in this instance - and it's not just with Verizon, by the way, we've written to all the carriers - is that it (network management) is moving from a technology and engineering issue to the business issues ... such as choosing between different subscribers based on your economic relationship with them."
AT&T Inc representatives did not immediately comment. A Sprint Corp spokeswoman said her company "goes to great lengths to be transparent about its network management practices," and will respond to Wheeler's letter as appropriate.
"Our network practices are consistent with the Commission's rules on the open Internet, are innovative and are good for consumers and competition," a T-Mobile US spokeswoman said.
Wheeler wrote those three carriers after receiving Verizon's response, an FCC official said. Verizon was the first under the spotlight because it was the only one to announce a policy change.
Wheeler is trying to establish himself as a strong defender of consumers and somebody who will punish Internet providers whose business practices run afoul of consumers' interests.
The FCC is weighing whether to set stricter rules for wireless networks to restrict discrimination or blocking of Internet content as part of new "net neutrality" rules that guide how broadband providers manage Internet traffic.
Sprint and T-Mobile continue to offer unlimited-data plans, but both Verizon and AT&T have discontinued them as all carriers try to shift their data-hungry subscribers onto tiered pricing plans that charge customers for specific amounts of data.
The FCC did not disclose the new letters, but the agency official said other carriers faced questions about their network management policies and practices that were similar to those faced by Verizon. (Reporting by Alina Selyukh and Marina Lopes; Editing by Leslie Adler and Paul Simao)