The rule in question might not necessarily be based on supposed liability (bike parts are available on line from 100's of outlets who simply sell parts and assume no liability other than for perhaps known or likely should have known selling defective parts) or any other conspiracy reasons.
It depends on where the co-op is located and who their clientele is. The two local co-ops that I'm familiar with deal primarily with either college students and/or those less privileged. They don't have 'must install here' rules but they go to great pains to assure that the part is the correct part and in proper working order. They assure that the buyer knows what installation tools are required and will urge parts buyers to bring bikes in so assistance can be given.
Does the average person even know basic bike maintenance? Would you allow someone else to even do a basic job like installing pedals on your bike and just assume they know the difference in threading? Moreover, not a lot of students or others have torque wrenches (ever over tighten a bolt or know someone who has?). Yes, some of you don't use torque wrenches and have never had a problem but...? Most people likely don't have basic metric tools.
There are many on these forums who question the competence of box store employees who assemble bikes...these are often co-op customers. It could be a rule to promote safety and no other reason. I'd bet that the rule isn't strictly enforced when a known to be avid cyclist wishes to make a purchase even though some avid cyclist have no idea how to wrench. 😕
Last edited by MAK; 11-22-21 at 12:03 AM.