Circumcision is a weird, arbitrary, archaic, totally unnecessary mutilation of a male infant's body. (Some) people support the notion for the same reason they support most things: they're accustomed to it. If questioned about it, they make up strange reasons to justify it....reasons which they have heard from someone else who heard it from someone else who heard it from someone else who...

"If there's an orginal thought out there, I could use it right now" - Bob Dylan

If they were accustomed to killing all their female firstborn at birth, they'd support that too. (and they do, in some places)

Oh, and the North American medical establishment supports it because it's lucrative, of course... Why would they discontinue an existing source of income?