You are right, the expression "throw away your vote" means "did not vote for my candidate" whoever says it. The expression is used as a means of mustering as many votes for one's favoured candidate as possible.
I know you are unaware of the election process in the USA (it's OK, you are not a citizen or resident of the USA so it doesn't matter at all that you are ignorant). We elect our president BY STATE. Generally, the winner of each STATE gets the electors of that STATE. You may not like that, but it matters not a bit since you are not a citizen or resident of the USA (and your country doesn't elect its Queen or PM by general vote of the people - by state or otherwise).
EVEN if there was a candidate who perfectly reflected all the views, ethics and philosophies of a voter (and it would be profoundly rare for that to be the case), such a candidate must WIN many states in order to be a viable choice. Even getting a plurality of the popular vote nationally doesn't not make for a viable candidate since we don't elect our president by national plurality of vote (just as Australia doesn't choose its Queen or PM that way). If he can't WIN enough states, he doesn't win. True, Mickey Mouse gets quite a few votes in every presidental election... but voting for him IS 'throwing away your vote' since he's not going to win, there is simply no path to victory (and I'm sure Mickey knows that).
Yes, of course, one might vote for Jesus (it's permitted to write in a name) perhaps just to protest the choices that exist.... and/or to make a statement about one's values.... and that's allowed and a few do that ... but it IS meaningless to the process. There will still be a winner in the state (unlikely to be Mickey or Jesus or you) and that one will likely get all the electoral votes of that state. Again, you - as an Australian - may not "like" this arrangement but it matters not at all, not a bit, because you don't live here (worry about your own country's inability to choose your Queen and PM by popular vote of all).
I DO think in 2016, Republicans could have steered all this toward a more acceptable candidate than Trump (I was rooting for Rubio) but that didn't happen, we ended up with Trump. And the Dems chose Clinton. It's how it works here. Again, you may not like how things work in this matter in the USA but it matters not a bit, not at all, because you aren't a citizen of the USA (focus on the reality that Australians cannot choose their Queen or PM by national popular vote), we ended up with Trump and Clinton. ONE of those would win (and that is what happened - no "third party" candidate won even one electoral vote). Again, you may not like this but that doesn't matter (you have your own Country to focus on).
- Josiah