With merchandise, it's one thing. The plastic is much sturdier and in it "for the long haul," per se. Cellophane on a poster can tear off far too easily and then the signatures are gone. Like Hardcore Legend said, all you have is signed cellophane. It has no value as a signed poster and you won't be able to sell it as such. I don't mean to imply that selling it is your ultimate goal; but should it come down to you selling it down the line, you could never say that it's a signed poster as it's not.

You could go and frame it which is a viable option. Then both the poster and signature are protected.

Me, personally, I would have taken the plastic off, had them sign the physical poster, and then framed that bad boy. But that's just me. To each their own.