I suspect that is what happened. Not knowing much about cars (although having had the shocks changed on my vehicle recently) I think that would be the most obvious explanation. But in this case the OP did everything right he asked BA if this would be ok. That is why I think the OP should place reliance upon the email he received from BA.

I suppose there may be an issue as to how clearly he explained what he was carrying to BA and whether or not BA could have been expected to anticipate that shock absorbers often contain compressed gas.

I am still not convinced that an airport authority has the right to destroy the contents of a customers baggage. If it cannot be carried and the customer abandons the item that is another story but the OP's father does not appear to have been given the opportunity to returning the item landside.