I don't understand what this supposed difference is. In what way are government aid programs optional that war and drug laws aren't? Why does it make sense to allow people to opt out of one but not another?
You say "clearly", but it's definitely not, at least not to me.
My objections to opting out of war policy and drug laws were practical. Nations must be able to fight wars. Drugs must be regulated (if only to establish civil liability). I don't see how an individual can practically opt out of at least funding those things. Humanitarian aid, however noble, is not essential like war policy, and it's not inevitable like drug regulations. I can go into more detail, but it's fairly off topic.
I am interested in finding ways to let people live their consciences in the face of unjust wars and laws. One way is to make more decisions at the local level and then people can move if they get really fed up with policies. This doesn't work with war policy, but it can certainly work with drug prohibitions and humanitarian aid programs. I don't see why the city of Tampa Bay couldn't legalize marijuana and send millions of condoms to southern Asia or southern Africa.
I agree that national defense is a necessity, but I don't think the ability to blow up huts on the other side of the planet is a necessary component to that.
Similarly, while I agree that regulating drug manufacture is a necessity, regulating drug consumption is not.
So there's first an argument from degree. It's not as simple as "Nations must be able to fight wars." Details matter a great deal, and I think a case could be made that even if war-fighting is necessary, the vast majority of what we currently spend on it is unnecessary and immoral.
Further, who decides what's necessary? I imagine a hard-core pacifist would argue that national defense is not actually a necessity. That same person might argue that access to good health care, including contraception, is a fundamental human right and therefore funding that is a necessity. Why should someone be forced to pay for anything they find immoral and unnecessary, even if you happen to think it's necessary?
My own response to that would be that some people are simply wrong about that, and allowing people to opt out of things they feel are unnecessary and immoral simply wouldn't work. The good thing about government is that it allows us to take collective actions that benefit us all but would fall victim to things like the free rider problem if they were funded voluntarily. But then we need some way of collectively deciding what's worth funding through government and forcing people to pay for, and what's not.
And this really just puts is back where we started. We have to fund some things, and no matter what they are, some taxpayers will feel that those things are unnecessary and immoral. That, I think, indicates that "I shouldn't be required to pay for things I find immoral" is not a good argument.
You say "clearly", but it's definitely not, at least not to me.