I can't give a great answer other than to say that streaming is weird that way. It's not unusual to find series that are available for free with commercials on some service or without commercials on a premium service. It's odd that AMC+ wouldn't have the whole series, but like I said, streaming is weird that way. I tried to search for "the wild west", but among the many results the only series called simply "The Wild West" was only available on Prime.
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