Let me start off by saying that I do respect your right to oppose the war, even if I do not agree with it.
As for answering the question/statement above, we should support the troops because they signed up to be willing to give their lives, should it need to be, for the freedom and protection of our country. Having been around the military as a dependent, and now working for it for 22 years, I understand how the army works and what sort of people are in it. Yes, they do get paid and good benefits. These are trade-offs for them putting their lives in danger. What kind of price would you be willing to put on your life (or freedom, for that matter)? It's not very fun to be put in harm's way, but these guys do it willingly to keep other's who don't feel called to or can't do it out of it.
As for this being a scheme of Bush, the facts behind the whole situation disagree with that statement. The people that our troops are now fighting have actually attacked us, and what we are doing now is keeping that from happening again. This group has made it well-known that they think all Americans are evil and deserve to die. They also believe that they are rewarded for killing us, especially if they themselves die in the process. If we just let them keep attacking us like they have through terrorist attacks stateside and on U.S. groups around the world, then we would basically be handing them everything the U.S. has on a platter. We can't do that.
There are also other people in Iraq and Afghanistan that actually LIKE the presense of our troops there and are greatful for the support and new freedoms they are gaining with the institutions of the new government there.
Also, supporting the troops takes on a very personal level for many people. Very often, you are showing support for one individual working very hard on a difficult and dangerous task. This person needs to know that you are behind him and believe in him for him to be able to complete what he's been given to do. You said that the soldiers knew that they had to go wherever they were sent. This is true, they did know that, though there are legal ways of getting out of a specific duty if you do not agree with it morally. It's the fact that they are WILLING to go anywhere to fight and protect people like you and me is the thing that earns them our respect and support.