I'm a willing participant at my workplace, but if the roof falls on my desk I have a case against my employer. The employer is required to maintain a safe workplace, comply with relevant laws and avoid unreasonable risks.
There is (obviously) no way that the players agreed to what actually happened to them, ie, banned substances, no records. Did they sign a document that said "We plan to trial some substances that might get you banned"? I think not.
This is a completely different issue to the question of whether the players wrong or idiots. I'm certainly not saying it was all the club's fault, or something like that. The anti-doping system can't work if athletes can offload responsibility on their coaches or support staff. However, both sides have responsibilities and the club certainly failed theirs.