You can go to social events which is the most appropriate time to socialize. There's meet-up groups, community groups, churches etc. that have social events outside of bars. You could always strike up a conversation with someone in a coffee shop, a grocery store, a book store, etc. You could also take some classes for fun and meet someone there. I met my SO when I went back to grad school, for example.

The reason it's less appropriate to approach girls when they're working, in my opinion is because:
1. They need to concentrate on their job.
2. It's a little rude to the other customers that you're taking up their time and distracting them, causing the service to be slower to other customers.
3. They're getting paid to do their job, not to fraternize unnecessarily with customers.

For most of these things it isn't really black and white whats appropriate or not. Because of the nature of one of my jobs, it wouldn't be appropriate to approach me in such a fashion because I need to concentrate on my job (I'm responsible for special ed kids and their safety and that needs my full attention). For my other job as a musician, I wouldn't mind as much and it would be appropriate and just part of the job. It wouldn't get them very far since I'm taken, but it would be more appropriate at the second job.