[NTG-context] Incompatibility catcodes of characters in ConTeXt and PlainTeX

Hello ConTeXist. Petr Olsak (maintainer of Czech/Slovak support for TeX) helped me with simple typing of sign charts. He made a PlainTeX macro who is working too in ConTeXt (see http://tex.stackexchange.com/questions/230273/typing-of-math-sign-charts). Petr Olsak (not ConTeXt even LaTeX user) was modified PlainTeX macro to work in ConTeXt and asked the question: "Does anybody know why ConTeXt sets catcode of & to 12? This means that it is incompatible with plain TeX". May I therefore ask , what is the reason that has character & catcode other than in Knuth PlainTeX? There are catcodes of other characters incompatible? Thanx Jaroslav Hajtmar