You'll notice, I didn't say that immigrants "founded" America. I said immigrants "helped build" what it is today.

I don’t understand how the motivations of pioneers and later immigrants are different. What do you mean by this distinction?

America was indeed founded by immigrants seeking a "better life" whether that was Puritans fleeing persecution or economic opportunists. It's not like anyone immigrated to America simply to "pioneer" for pioneering's sake.

Of all the groups that immigrated to the US the Puritans would be about the last group I'd say anything positive about.

By that standard you could say that every decision made by everyone is made in pursuit of a better life. Which is true, sure, but it obfuscates certain arguably important points.