I don't think people are ignorant of whether or not Mormons believe in Jesus, so much as they have a narrower view of what Christianity is. Many evangelicals don't consider Catholics to be Christian, and Eastern Orthodoxy is as foreign to some as Islam is.
And I mean, I can see the point, too. The tenets of LDS differ greatly from the tenets of mainstream Christianity. This isn't like how the Orthodox and the Catholics argue over whether the Pope is the boss of the Patriarch of Constantinople or whether the Holy Spirit proceeds from the Father alone or from the Father and the Son. There's a whole extra book in play here and a whole host of other major theological differences. I mean, I would compare that to the difference between Judaism and mainstream Christianity or mainstream Islam and the Druze. There's no doubt the latter developed from the former, much like the Latter Day Saints developed out of mainstream Christianity, but I get upset when those Jews for Jesus people call themselves Jews. They're just Christians who don't like bacon for some reason. Sure, they still believe in the Jewish god, but I don't think that alone makes them Jews. I think same goes for the belief in Jesus.