Because while you are technically innocent until proven guilty, society treats folks that are arrested as guilty unless proven innocent - and even then, if they were acting properly, they think they wouldn't have been arrested.

Example: a background check sometimes produces an arrest record (not a conviction), and that can cost a job. Background check companies don't have to be licensed, and no one keeps track.

But you are correct: The burden of proof should be reversed. We know most folks aren't a flight risk and gentle measures - such as having folks check in and calling folks to remind them of court dates - have good results.