The poster is for marketing purposes. It doesn’t display the actual title. What’s in the movie is the title, which is: Star Wars Episode VII The Force Awakens. It goes the same for every other episode since 1980 (TESB has no number on posters but its title is Star Wars Episode V The Empire Strikes Back).
I still don’t get why people don’t want it to be part VII (well, I don’t want either it to be part VII but it is…).
Even in 1980, Episode V was in the title? I’m aware that it’s in the crawl, of course, but I didn’t think that was “officially” part of the title until around 2004 when numbers were slapped on everything.