there is a line which is crossed, one where there are legitimate theories, then just the nuts that think everything is a conspiracy theory.
this is hard to define.
let's look at the gulf of tonken for example.
The US said one thing, the vietnamese said another thing.
Turns out, they were telling the truth and we had a "mid level mgmt mistake" about being attacked. It led to a conflict that our leaders wanted, that lasted a long time with much cost in blood and trasure.
Papers declassified years later showed this. Was there a 'conspiracy'? well, we went to war over a lie. People who said we were not attacked were called conspiracy theorists, as it directly went against what the govt said.
If the USA, in the 1960s, was willing to lie about something so big to get us into a war so big - it's not that crazy to look at today's events and question some things. An entire war for a decade - built on a known lie. Unbelievable, isn't it?
http://en.wikipedia.org/wiki/Gulf_of_Tonkin_incident