Without knowing the original terms of purchase I can't say whether extra pay for thinning is inappropriate or not. If thinning was included in the purchase price and contract then yes, it would be crazy. Paying on the basis of current board footage owned by each individual is impossible simply because there will be owners who refuse to participate. In addition, a costly timber cruise on each stand would be required before each owner's individual cost could be calculated.
In my experience (in the U.S.) the cost of thinning (commonly known as "timber stand improvement") is calculated on an acreage basis or is competitively bid for the entire project. The problem here seems to be that the trees are owned by individuals, rather than the company responsible for their management. I just can't see how that might work.
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