Hmmm, I must be completely clueless too because I collected a handful of shells in December. I didn't see any signs about not taking shells and I didn't hear an annoucement. I do specifically remembering the sign about warning about sharks so I am surprised i didn't see the sign about only leaving footprints.
Lots of people were collecting shells on the beach and no one reprimanded anyone.
If I see any shells on my upcoming trip, yeah I will probably collect them. As long as they are empty shells. I would never take shells with a living creature insides. Perhaps Disney is just worried guests will take living creatures with them because I did see plenty of starfish etc.
I wouldn't buy shells for a souviner because who knows where the shells in prepackaged bags come from. The only reason I would take a shell is because I want a souivner of a specific beach, not some random shell from who knows where.
Feel free to flame me because my opinion isn't changing.