I don't think they bought copper membership after became legendary. Like me, I bought copper membership once I was newbie.
copper membership was first introduced in November 2017, roughly about ~44 weeks ago
(44 weeks/2 weeks) * 14 activity = 308 activity and to get Legendary minimum 775 activity
lets assume merit isn't a problem, and a user bought copper membership back in November
so if that user is currently a Legendary then he should be at least Sr. Member when he bought it
775 - 308 = 467 activity, close to 480 for Hero Member
After become legendary buy copper membership means waste of money.
if current Legendary member has copper tag,
surely for vanity reason as the copper benefits are no use for him with higher rank (when he bought it)
or he paid for it
(as downpayment) for possible future developments
I wrote the system so that I can easily add additional paid memberships in the future, but I might not ever do so. Not sure.