I think you are disregarding the use of synonyms to use a word in a different context than that which it is described as, ergo enlightenened can mean to educate rather than shine an actual beam of photons.'illuminate' means to cast light and 'enlighten' means to cast knowledge, you can't cast light on knowledge because it is n't tangible but a smile can illuminate a face for example.
To interchange the two is a very lazy use of English.
Appropriate synonyms don't usually bother me, except that one. It really pisses me off because it's so obvious but folk still use it to come across as having a vocabulary but don't realise how ignorant it makes them look when they do.
A quite funny explanation is.......
'Illuminate' is when somebody shines a torch in your ear.
'Enlighten' is when your the torch.