Another case where someone should think twice about this, is if you work in the rail industry or associated employment. Quite apart from the working hours issue, it would be very easy to fall foul of conflict of interests. If someone like myself, who works for Network Rail, decided to build a route based on an operational UK prototype I could see some searching questions being asked by my boss as to whether any data used to produce that route had been obtained through company sources (e.g. the portal which has line diagrams and gradient profiles), or even just in company time (perhaps a quick glance at Wikipedia or Open Street Map during a quiet period). You could probably rinse and repeat that for other countries, too.
Now as it happens my particular penchant for route building tends to be strange and unusual, ranging from obscure Yugoslavian narrow gauge, to the outposts of northern Europe, Siberia and the deserts of Eastern China (to name but a few). However even then I would still be very wary about putting a small second income against my primary wage and pension. Anyway, in truth, I could probably earn more working a couple of rest days (16 hours) than the 160 hours or more it might take to produce a half decent route to commercial standards. And it's all PAYE, no worries about self assessment or N3V trying to claw back twice the money they gave me over some technicality.
Re Paul's comment about grounds for a refund, don't forget Steam now offer no quibble refunds if you buy something, don't like it and request your money back within a certain timeline. N3V may well have something like that in mind, or may be forced too when people start objecting to paying £25 for some kid's oval of track round a couple of baseboards. Even without that, there's Paypal disputes or Debit/Credit Card chargebacks...