That could be said of those who "didn't realise", but I would not say that there would be any "hardness of thinking" on the part of someone who understood full well what could be done in compliance with the regs but didn't want to have to possibly deal with the 'hassle' of explaining, maybe to someone with little technical or regulatory understanding, why it was acceptable to apparently 'go against' the guidance of the Appendix.... so the hard-of-thinking who don't realise that the Appendix cannot 'forbid' anything, or who realise that but want to avoid the possible hassle of having to justify "contravening Appendix 15" essentially think of it as if it were (and could be) 'forbidding'.
Kind Regards, John