The designer said
it is optional.
As does the rulebook
page 15 wrote:
Recharge: This explains circumstances under which you may recharge the card
When playing Lem I would often prefer not to recharge the Cure spell so I can get it back to my hand.
Just to be clear. The Recharge section of a card is optional.
If you are using a power on a card (or a character's special Power) that starts something like "Recharge this card...." then it is not optional.
Example: Wooden Shield "Recharge this card to reduce Combat damage dealt to you by 2".
Example: Dagger "you may additionally recharge this card to add another 1d4".
I know you specifically mentioned Lem recharging the Cure spell but the above two are just examples. I wouldn't particularly expect Lem wanting to discard those cards instead of recharging them