By convention replies are sent to the thread root's defined "read" relays.

I think it's fair to assign responsibility to the thread creator for picking unsafe relays. But of course it could also be your client's fault for using random public relays or something like that. Relays are the only way to filter content properly on Nostr, clients and users should start to be more mindful of that.

In my case I use Gossip with the nuclear option of only reading replies from the relays I specify manually, so even if the thread creator is evil I won't see evil replies, but this is not the ideal solution either.