The "voluntariness" of reserve service is greatly misunderstood by many people including senior military leadership. I used to give lectures on this to COs of both the reg and res components and for the vast majority what the full impact of the law is was an eye opener for them. It's no surprise that the folks on the parade square don't have the full picture.
Effectively, a reservist, like a regular force member, volunteers only once - when they enlist, are enrolled and take the oath. From that point on they are subject to the Code of Service Discipline in accordance with the terms set out in the NDA and all the provisions of the NDA. That obligation continues until they are released.
There are dozens and dozens of liabilities that come with service. NDA 33(2)(a) is the one on being liable to be ordered to train and QR&O 9.04, as mentioned above, states how much training a CO may order.
IMHO s. 294 is one without teeth if it is not being used, and quite frankly, I encouraged COs to use it as an object lesson pour les autres. IMHO, the "come play when you feel like it" attitude that pervades the reserves is one of the key reasons why, as an organization, it is so ineffective. It's close to impossible to create a unit that has cohesion at the collective level if only a third of the folks show up for training.
The problem, again IMHO, is that no one at the senior command level is prepared to make an issue of it to change the system as long as they get the thousands of Class B's they have become addicted to. For them, Class Bs are a feature. To anyone who realistically looks at the mobilization capability of the Class As, Class Bs are a bug.