The pope used to have the inverted on his nice hat. It's known as St. Peter's Cross because Peter begged to be crucified upside down, feeling unworthy to die the same way Christ had.
An upside down crucifix - with a representation of Christ - isn't the same as an upside down cross. An upside down crucifix is meant to mock Christ. This is the reason St. Peter requested to be crucified upside down - he didn't feel worthy to die the same way as Christ. Even then, an upside down cross is meant to symbolize the Antichrist, not the devil.
Satanists are free to employ any symbols which they feel have resonance, so if such an upside-down cross has personal meaning they could use it. However, the prime symbol for Satanists is the Sigil of Baphomet. The symbol to the right is the alchemical symbol for sulphur. The stench produced when this substance is burned has long been associated with the Infernal regions. It has no other significance despite some strange interpretations by evangelical Christians.
“The devil…that proud spirit…cannot endure to be mocked.”
So really, we're actually fighting him.