To answer the first question we might also ask: Is it strange that a man could rise from the dead, walk on water, heal the lame, create the heavens, and answer the prayers of billions of people? There are basically two kinds of non-Mormons who reject LDS beliefs: A) those who believe that there is no God (or that if such a being exists he doesn't interact with humans), and B) those who believe that a supreme being exists and has communicated with mankind.
For those who don't believe in a God, all supernatural and miraculous events are automatically brushed aside as imaginary, impossible, etc. All spiritual experiences are seen as "strange," superstitious, and possibly the result of the evolutionary process of the mind. Joseph's translation process is just as strange as any other supernatural claim.
For those who believe that God can and has communicated with mankind, it seems hypocritical to summarily dismiss Joseph's method of translation because it doesn't fit with pre-conceived views of how God communicates. As with all spiritual claims, the only way to know if they come from God is to ask God for a witness.