I also have trouble understanding how bitcoins can be seen as property.

I understand people feeling possessive about the access key they've found and the influence those keys have on the network. I also can somewhat understand that people are willing to try to convince (with money) others to transfer some of that influence. But no single person "owns" Bitcoin(and as a result bitcoins) and if one single person did, that would mean Bitcoin has become worthless.

My guess is the IRS couldn't be bothered and picked one of the most used metaphors, probably the bitcoins are like gold one, and then thought: Ok gold, check, that was easy, next.

My guess is that the emphasis on using metaphors has come to bite the US-bitcoiners in the arse.