Not to mention that most Apple devices can't harm anyone else - i.e. there is no third-party liability that Apple may have to assume (or buy insurance to cover)

Not to mention that most Apple devices can't harm anyone else - i.e. there is no third-party liability that Apple may have to assume (or buy insurance to cover) if you (a free 'test driver') harms someone else with their test product. This $300 is definitely not unreasonable.