It’s actually surprising that it took Bill Sienkiewicz this long to find out that 20th Century Fox had used his artwork, as the Dazzler cover art actually didn’t even appear in the theatrical cut of X-Men: Apocalypse.
However, the folks behind X-Men: Apocalypse seems to have forgotten that — and one artist is certifiably pissed off.
Artists generally receive some sort of credit when their art is used in any capacity; it’s just the nature of the beast.
However, it’s still always important to assign credit when it is most certainly due.