I find it very interesting, and not a little ironic, that Snape--a man whose very life has been shrouded in lies and broken 'promises' for years should have a such an issue around Harry's breaking his. It also interests me that Snape *did* leave all his things lying around for Harry to snoop through, *knowing* how much Harry wanted to save him, and how inclined Harry has always been to cut corners and break rules in the pursuit of what he thinks is right. It's almost as though Snape wanted him to 'break his word', but couldn't bring himself to give outright permission to use those memories.