My guess - and it is a guess - is that there may be some property rights issues involved. If we're not talking about raw figures, the accountant may have already done some work to format/analyse the data; so the figures embody an element of the accountant's intellectual property. Thus, if the agreement was terminated mid-way through, the accountant would be entitled to some payment before returning the data.

The link below is not strictly relevant, but it does highlight some ownership issues when data are handed over.