They do collect taxes (Prime subscription, cut from sellers), enforce compliance (sellers can't sell cheaper elsewhere), provide infrastructure (own delivery network), and "nationalize" services (start selling common items from Amazon Basics while delisting the originals).
It's always felt weird to be that people in favor of small governments want to let them go rampant.
I would be one of those "small government" people. And it's not so black and white. Just because we see private organizations becoming huge behemoths running rampant in the market, doesn't mean that we want that from a small-government perspective. If anything, this is 100% a failure of government if we collectively agree that Amazon shouldn't be that big or behaving in a certain way. Why haven't we had a referendum about breaking up Amazon? Or any other hot-topic for that matter. We barely have one every few decades and still can't seem to make our minds up (e.g. Brexit or the Catalonian referendum for secession).
Right now, there are huge swaths of government laws and regulations that prevent the market from adjusting to Amazon becoming so large. Even something as simple as copyright laws that affect the legality of scraping data has an effect of propping up Amazon. You have to look N-levels away from Amazon and their lack of competition to appreciate how we are effectively promoting/allowing something like Amazon to occur.
It's always felt weird to be that people in favor of small governments want to let them go rampant.