"The tragedy of the commons is a situation in a shared-resource system where individual users, acting independently according to their own self-interest, behave contrary to the common good of all users by depleting or spoiling the shared resource through their collective action."

In this case, given that everyone else is going to use plastics, it is in my interest to use them too. But I would be better off (maybe) if noone used them. Does that not fit the definition?