A question with bounty

I will give 0.019 steem to the one who can answer my following question with the best explanation. (Please don't think that the reward is too small)

I've been being a fan of ONE-PIECE since I was a secondary student, I really love the devil fruits setting. It gives me an imagination on it. The one who once eats a devil fruit, he will receive some special power. However, there is a fatal weakness for devil fruits users, which is sea water. Sea water seems to be harmful for them, so they can't swim in the sea or even touch sea water. In the manga, some people use seastones to catch devil fruits users to counter them.

Then, a question's coming, have you ever thought why all devil fruits users are determined to be pirates? Actually, in the beginning of ONE-PIECE, there are pirates living in mountains. Why they do not be one of them but to build a ship and live on the sea, isn't that strange and unreasonable?

And it's time for you to comment to give an answer below. I hope I can get rid of this question.