Called unsporting for not warming up on one side of bat.

Last night in a local league I was called unsporting for not warming up on a certain side of my bat (antispin rubber).

In 25 years I've never been ordered to use a side of my bat for warming up and was called unsporting by my opponent and using a cheap tactic to get early points in the game.

Is this a rule? I understand that rubbers can be inspected but it's the first I've heard of this.

Edit: For clarification, the opponent had watched me play two games using antispin rubber and was aware because I had already told them.