Anarchists are part of the left. I think you're thinking of anarcho-capitalists who reject that sort of coercion. Which is not to say anarchists aren't against Obamacare, but it tends to be [0] more on the grounds that it's corporatist incrementalism. They're against "pay or die" coercion in general, and the capitalist healthcare systems as built on that premise.

[0] weasel words used as I am not an anarchist nor have I read a substantial amount of anarchist theory

I'm a little fuzzy, also. I assume anachists would not be in favor of a law like Obamacare because it was imposed on the 49% of the population who were strongly against it. I would assume acarcho-capitalists would have the same conclusion.

I understand the historical relation between anarchy and other leftist politics like socialism and communism. But since anarchists seem to be opposed coercive top-down hierarchies, idealogical or otherwise, I found it interesting that they are considered to be on the left and not outside of left/right politics.

As I'm typing here, headed towards looking up the differences between left and right, I found this: "In both its social and individualist forms, anarchism is usually considered an anti-capitalist and radical left-wing or far-left" [0]

That jives with the overlap I've noticed between anarchists and a subset of libertarians.

[0] https://en.wikipedia.org/wiki/Anarcho-capitalism