Hidden Love
I'm going to go out on a limb here and make a wild claim. I think that Steinbeck intends for their to be a secret, hidden love between Al the fry cook and Mae the waitress. It just seems to fit. Al seems so protective of her, he works hard to help her earn her tips from the truckers and seems to always be paying attention to her. Steinbeck adds a line of jealousy when Mae flirts with the customers for the tips. Mae at the same time seems to do whatever Al says. She doesn't always like it, but she's willing to sell the bread for cheap and even lie and sell the children candy in order to please Al's wishes. Neither character has a critical role, and I'm not sure yet why Steinbeck would bother with it, but I do think that the two have a special bond that goes deeper than coworkers.
I thought of that too when I was reading. It seems like Al, wanting business, isn't going to say much to discourage Mae from her flirting, but he still likes her.
And Mae could only be flirting with the truck drivers because she knows they'll give her tips and return several times. Also, it is only Mae and Al in the diner all day long so they must do a lot of talking or something when there aren't any customers.
Al could also be acting like a father-figure toward her and he's protective of his girl. If she likes one of those drivers or one of them snatches her away, then who does he have to help him and be the pretty face behind the counter.
Haha, this is funny because I was thinking this too. I don't know if I totally agree with Aja though, I'm not really too sure that there were any fatherly undertones going on, but it's a good way to look at it.