I think it's probably perfectly safe but I think getting food out of a bin and then serving it up to other people without telling them* is actually pretty offensive. Even if
you're satisfied that it's ok and can't see why anyone else would think different, it's not normal acceptable behaviour and by serving it up to other people as normal food you are imposing your own, low, standards on them, unwittingly.
If you don't agree with supermarkets' policy on throwing food in the bin, then surely either getting a managerial job there or buying up a bunch of shares and going to the AGM and effecting policy change from within is far better than just going and getting it back out of the bin again.
*If you explain and give them the option, then that's a different matter obviously.
edit: Anecdote (1): I once had a job in a cake factory where one of the tasks was removing labels from cakes that had not been sold and replacing them with labels with later dates.