Semantically, I personally use "due to" and "because of" interchangeably, and I think it does a service to treat them as such. "Due to" is just one less syllable than "because of". "Due" really means "owed", so if anything it makes sense to "owe" for something good, not bad - but in practice, they mean the same thing. Now, syntactically, there is a very subtle difference that is often ignored in common speech: "His defeat was due to the lottery issue." "He was defeated because of the lottery issue." I found the above example online, and it shows how to "correctly" use these phrases. "Due to" introduces a predicate adjective phrase (meaning it is used after the copula "being" verb), while "because of" introduces an adverbial phrase, meaning it modifies a full verb. Like I say, though, many native speakers do not often make this distinction.

Both "because" and "due to" are used to introduce the reason for something. The only difference is that "because" is used to introduce all kinds of reasons, whether they are good or bad. By contrast, "due to" is mostly use to introduce the reason for an undesirable situation. Sample sentences: Because he helped, I passed the test. (O) Because there was a traffic jam, most of them were late. (O) Due to his help, I passed the exam. (X) Due to the traffic jam, most of them were late. (O)