That doesn't make sense. Russia can use this as a propaganda tool without having to act on it. And it's not like that Russia didn't accuse USA from blowing it up - they did, just not very forcefully. It's strange, because they could have used it in e.g. Germany to split the society: "bad Americans want you to freeze, they blew up your gas pipelines".
To acknowledge such an attack and not respond in kind would cause them to lose face, but responding in kind risks escalation. Ignoring the attack is their safest option.