[thephyber]

> The cop is already pretty sure you did it (or he wouldn't have brought you in)

There are different stages before arrest that matter here.

There is a burden of "reasonable suspicion", "probable cause", etc. (or some equivalent like "exigent circumstances"). An arrest requires both of these, plus some evidence of a crime. Being convicted at trial requires "beyond a reasonable doubt" (for criminal charges, as opposed to {civil, tax, military}).