Answer:
Some Americans do this because as time went on, the English language started turning more and more in the direction of slang. "For certain" turned to "for sure," and now some Americans say "shore." This could also have something to do with the section of the USA that they are from- Southerners tend to say "shore" more that some other Americans.
Step-by-step explanation:
I dunno if this question was just you trashing people who say "shore" or if you actually needed an answer, so I just gave you what I think would be the answer. ;)