פירוש על עבודה זרה 42:3

By implication, renting a bath-house to a Samaritan should be permitted. But while Samaritans observe the Sabbath and do not work on it, they do work on intermediate days of the festival. The assumption here is that Jews do not work on these days and thus people might think that he is working on behalf of the Jew.

The answer is that Jews also are allowed to work in a bath-house on hol hamoed by heating up the water.