I'd agree in that case, mostly because the code does not have a restrictive license. Also, to me if you disclose that you are using yacc then you are being totally up front and have side stepped any issues. But in the case of like byacc the license specifically states you can use it in commercial or public code without restriction, so it is fair to use in my opinion.

Me personally, even in like this case where it isn't required or requested, I'd likely attribute the author or repository in my license file, credits or readme.