Alright, maybe I need to make it more blatant for you that you need to actually address what people are saying, not what you're imagination is telling you erroniously is fact.
If you have no got the message yet...you are strawmaning. Its a personal pet peeve of yours. Wonderful, great, deal with it. Until you can show some kind of definition or rule that clearly states that the use of the world "Party" as a capitalization in ALL cases refers to a nationally identified political party your pet peeve is nothing but your own inability to deal with your opinion not being fact.
It became known as the "Tea Party" movement as a throwback reference to the Boston Tea Party (note, the Boston Tea Party was not a political party either). It is capitalized as it is a formal, organize...albiet loosely...group and as such gets formal recognition. However, there is few...if any...that are part of said group that actually suggest that its the "Tea Party" Party, as in a political party.