Do recommendation systems necessarily use machine learning algorithms?
There's nothing about a recommendation system that absolutely necessitates some kind of machine learning. Indeed, I've seen decision systems in use that were essentially just someone's idea about what the customer's preferences ought to be.

A recommender can be based on anything from a few ad-hoc 'common sense' rules, to a logistic regression someone did on some data a few years ago and whose parameters are hardcoded into the system, to a complicated ensemble of machine-learning algorithms that are regularly and constantly trained on new data.

The use of machine learning for recommender systems is partly driven by necessity, partly by fad (at least from what I have seen). If a simple recommender works well, and accurately predicts what the user wants, there's no need for a machine to learn anything. If there's a huge amount of data, hiding some very deep relationships that humans are unable to pick out, that's where machine learning becomes useful.