Maybe so, but it encourages employers to find ways to circumvent minimum wage laws. An sufficiently creative employer could find ways to have employees on-site

Maybe so, but it encourages employers to find ways to circumvent minimum wage laws. An sufficiently creative employer could find ways to have employees on-site for 8 hours per day, but only classify 4 of those 8 hours as paid work and find excuses why the remaining 4 hours are off the clock. E.g. setting things up, putting tools away, doing paperwork, etc.