Why not? vim indents one more when encountering a colon at the end of a line. It also de-indents when encountering a return or break statement. Of course in all other cases you have to de-indent manually, however, hitting backspace once is not more work than typing a closing curly brace.

What I mean is that you can't tell your editor to indent an arbitrary block of code.

Right, because a block of un-indented python is syntactically incorrect. It would be just like stripping all the brackets out of some C code and asking the editor to add brackets to the resulting mess properly.