All free relays used to be good at the beginning but since Nostr is gaining traction the bots and spammers are coming. Ive seen different type of spam on nostr mom. It's recommended because it was good at the beginning and it's a default relay. You can change the default relay but then spammers will just move on that relay.

The developers want Nostr to grow so they want to keep the relays free because they say if there is not free relay, Nostr will not grow. No matter what free relay there is, spammers will attack it.

Once you have been on nostr long enough you can move to the paid relays. That's the best solution to avoid spam.