You are not the first one to point this out. Here people don't think about the US government, but the left party itself. I personally find this theory unbelievable. Even the US government theory doesn't make sense, for Aznar supported GWB on the war, and everybody knew that if the left party got into the government they would withdraw the armed forces from Iraq. People took the bombings as a retaliation from terrorists for Spain participating in the war, so people didn't vote for Zapatero but against Aznar.
It's clear that the purpose of the bombing was to force a change in government. GWB didn't benefit from it, but the terrorists did. And it also send a message worldwide: "look what happens when you attack us". There was a special comission afterwards to investigate what happened before and after. Sadly, instead of knowing the truth, politicians ended up pointing fingers to who knew what and who kept silence.
Raul