Well no, actually the worst case scenario is that, should the author later wish to license his work to a company which wishes to produce a commercial game, the mud/s will be seen as competition for that company, and will therefore threaten the authors chances of realising a profit.
However you're quite right in that many authors are willing to give permission. Just be careful though, because many will also reserve the right to withdraw that permission at a later date (meaning you'd have to scrap your mud).
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