The mobile-payment service, run by Twitter co-founder Jack Dorsey, amended its terms of service for retailers last week to ban sales of "firearms, firearm parts or hardware, and ammunition; or... weapons and other devices designed to cause physical injury."
A Square spokesman declined to comment on this specific change, but said: "From time to time we revisit our policies governing the use of Square to ensure they are consistent with our values and in the best interests of our customers."
He denied that the move was related to the recent debate over gun control in Washington, calling the timing "completely coincidental."