Who decides the paper acceptance rate at a workshop organized as part of a conference: the workshop organizers or the conference organizers?
This can depend on the conference. My experience (CS, various ACM conferences) is that the workshop committee is responsible for its own papers and agenda. 

But the workshop has a limited time frame, possibly determined by the conference committee (time * space = money). This can have an indirect affect on the rate of acceptance since it implies a limited number of papers that can be accepted. 

So, it isn't a decision that, say, 10% of the papers submitted will (likely) be accepted, but that there is only room for 10 papers, no matter how many are submitted. Some others might get shunted elsewhere, such as to a poster session. 

But I assume that isn't universal and can hypothesize about a number of different possible arrangements.