Once, when I was leaving a cinema after the Hong Kong premiere of a Hollywood film, I noticed watermelon seed shells scattered around a seat in the VIP section. It reminded me of those bars where peanuts are served and the patrons are encouraged to throw the shells on the floor. But, really, come on - there is a time and a place for everything.
A glamorous film premiere is not the right time to be munching on watermelon seeds, although such fare is acceptable outdoors, in teahouses and at a raucous Chinese opera performance (although it's still courteous to clean up after yourself). For one thing, eating watermelon seeds can be messy and, secondly, the cracking of the shells could be annoying to the people sitting nearby.
Getting through the shells of watermelon seeds - as well as other types, such as squash and sunflower - to the tiny treat inside is a lot of hard work for very little pay-off. There are people who eat the whole thing, indigestible shell and all. I guess the advantage is that they are consuming a lot of roughage. The seeds are also sold without the shells, which makes them effortless to eat, although this means you miss out on the salt (or other seasonings) that the complete packages are flavoured with.
It takes practice to extract the whole seed from its shell without crushing it. Each shell seems to have a "sweet spot" that cracks it cleanly, just enough so you can pull out the edible part with your teeth.