I am with riverbee on this. The moisture and high sugar content of the melon when there is nothing else, is probably the attraction. Do you still have supers on? There may also be some minerals that they are receiving as well. In my own backyard, I have some asters that I started from seed last year. I put them in 6 inch pots last year and over wintered them in the same pots. They are still sitting in the same pots and same potting soil. I have them on a table in the sun so they require daily watering. My bees crowd around the drain holes of those pots. I have birdbaths which they frequent and there is a city lake next door, but they are drinking from the drainage holes for a reason. I think it's the minerals. I saw this last year with something else I had potted up.
Just a thought.