Any person regardless of nationality can be compelled by the nation state in which they are physically located to serve in that nations armed forces. This is because that person is within their dominion. While uncommon, unwise and rarely done, it has been done by nation states. This is because the nation state can force the individual to serve or suffer punishment.
If Mr Cruz were to be “compelled” by Canada and is not within their dominion, he can renounce his citizenship and then Canada would no longer be able to force his service as they would not be able to enforce their laws.
Again, US law governs US citizenship. Looking at any other nations laws is speculation of a hypothetical.
You left out "unlawful". You are simply trying to dodge the point. If Mr Cruz had remained in Canada, and had been drafted into their army engaged in a war against us, after all was settled, no one in this country would make any objection to Canada for their having used Mr. Cruz in this manner.
Should they do such a thing to you or I, the State Department would consider it a violation of international law, and would likely bring it up as a charge against that Nation.
In Cruz's case, we would recognize the right of Canada to do so. In your or my case, the United States would regard it as a breach of civilized norms, and a crime against the United States.
Stop pretending that there is no difference. There *IS* a difference.