That's why it's disclosed as a provision. It's something frequently used by companies where bad news relating to old events is coming somewhere down the line, but can't be quantified exactly. Under accounting rules, you have to provide something, so you use a best estimate. Pretty much all trading companies, for example, will carry provisions for bad debts and unsaleable stock. You also have to, for example, provide in full for the cost of planned redundancies, although the exact costs won't be quantifiable at this stage. Once Newcastle had identified that there were future obligations arising from these onerous contracts, the rules demand that the costs have to be provided for, at least to the extent of the best available estimates. If the provision turns out to be overestimated, the excess will be released to profit later. On the other hand, if they've been underestimated, there could be more losses taken in the future.
They're not too common within football, but they're far from unique. What happens more frequently is that clubs will make provisions against the remaining contract value for players who won't command a fee, and are surplus to requirements. Sunderland wrote off some in 2016/17, and ought to have written down a whole lot more in 2016/17.
The general rule in accounting is that you should recognise losses in full as soon as you become aware they'll happen.
As an aside, provisions are a fruitful source for creative accounting - if you have profits in one year which are embarrassingly large, it can be useful to find something you cam justify a provision for to the auditors to get the profit down a bit. Lo and behold, a year or two later, you suddenly find you don't need the provision any more - usually when you could do with a bit more profit.