It's for this reason: A politician is supposed to be somewhat of a role model, someone whose actions you can follow. They make our laws, after all. Now, I know that 90% of politicians out there are crooked, but most of the time, the ones that get caught are the ones that get impeached/resign/etc.
Nixon resigned for Watergate, several presidents have been impeached (Clinton included), and countless mayors, governors and congressman have resigned or been inpeached for lying, cheating on spouses, etc.
Now, on the other hand, if a politician had cheated or lied BEFORE getting into office, that's a different story. If the populace is dumb enough to elect him, whatever. BUT, if he commits those crimes/bad judgements while in office, that's a different story. He's an elected official, and should behave accordingly.