Not to mention they need those spell slots to fight too. The grove has been getting harassed by various monsters ever since the tieflings showed up. Feeding the tieflings literally makes it harder for the druids to defend their home. As anyone who's played a Moon Druid and had to sacrifice a perfectly healthy wildshape form to cast goodberry to save a party member can attest to.
Based on what I've seen, the Druids aren't even doing the defending. It's the Tieflings actually fighting, the Druids are hiding.