The practice is that if they employer docks 100% of their workers' pay - as they have a right to do but most are choosing not to inflame the situation further - then they legally have to send you home; in effect to 'lock you out' of your duties. Employers cannot be disciplined for participating in mandated industrial action. It's true there are areas of grey here, but technically this is not a breach of contract.
The 'voluntary' disclaimer is just that: a disclaimer.
Only the more draconian employers are choosing to impose the 100% deduction. Most have gone for a smaller percentage. And whilst it's true all professions require an element of goodwill, ask most academics and you'll get the picture that many work about a 60-70 hour week when their contracted hours are anywhere between 35 and 38. That is a LOT of goodwill. It has now been lost.
Working to contract (ASOS, ie action short of a strike) means doing merely those 35-38 hours and no more. A good many employers have threatened 100% deductions for ASOS too - for simply doing our jobs as contracted - that was long before MAB was even a serious prospect. I would hope someone in the union has questioned the legality of this.
Yes, there have been illegal practices abounding since this phase of the dispute started, including management hounding individual employees about their intentions. This, at least, is black-and-white illegal.
I think you can credit educated academics with an understanding of how the terms of their employment work, and what the consequences of industrial action are. We do so advisedly, not on a whim. Nowhere have I said I don't have the courage of my convictions or their consequences. I do. I've knowingly accepted the 100% pay deduction. This doesn't mean that, as a principle, I don't think it immoral.