Imagine it is your job to ensure safety in the stadium, and you are legally responsible for taking any reasonable measures to safeguard everyone within it. The risk of someone luzzing a full bottle and that causing injury has been identified, if there is a reasonable way of reducing the risk, you have to take it. Remove caps from bottles and the risk is reduced, you have done your job. Then you are made aware that some people bring caps with them to get around your safety measures, if you do nothing with that information and someone gets hit with a full capped bottle, you are open to a few problems coming your way. If you had issued a statement as the club have, and instructed stewards to remove bottle caps if found, you are probably going to be ok.
Afraid it's a rubbish argument as lobbing change at players will cause more injury and they aren't going to ban supporters taking that into the ground.