[slaymaker1907]

It's really not playing nicely to create a closed frontend to Prolog implementations which are open source, particularly the GPL ones. It seems like they aren't violating the licenses of those Prologs, but just because something isn't against the law doesn't mean it is ethical.

[teraflop]

Out of curiosity: if somebody developed this front-end purely to satisfy their own curiosity and never released it, would you consider that to be unethical? If not, how is posting the source code without an explicit license grant worse?

[yowlingcat]

Could be an excuse to get it nebulously embedded and then retrofit a more restrictive license in to benefit from lockin. Bit of a reach, I suppose.