Reverse engineering for compatibility, and implementation of a compatible system (as long as you don't copy the original) are not just legal, they're explicitly legally protected in many jurisdictions. You'll get in serious trouble if you copy the original, but there is specific case law supporting things like emulators. See, for instance, Sony v Connectix and Sega v Accolade.

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gmueckl 4 hours ago | parent [-]

But OpenTTD is explicitly a faithful copy of the original. It replicates the original product in appearance and behavior and is open about it. If you were to dig into source code history, mailing list archives, chat logs etc. I'm certain that you could find a lot of evidence to support this position.

	▲	JoshTriplett 4 hours ago \| parent \| next [-]
		"Behavior" isn't copyrightable; it explicitly isn't, in fact. To what extent did they copy "appearance" other than supporting the use of the original assets? It is certainly possible that they didn't scrupulously maintain clean hands, but I wouldn't automatically assume that.
	▲	einr 4 hours ago \| parent \| prev [-]
		GNU’s Not Unix is explicitly a faithful copy of UNIX. It replicates the original product in appearance and behavior and is open about it.