Hm, I thought a lot of episodes did have transcripts? I've read some before. Here is a fairly recent one. Admittedly the coverage is not consistent.
As others have said, I think it's really difficult for either party in written communication to be sure of the other's intended and perceived tone so there is potential for misunderstanding. My instinct initially is that one could indeed argue your volunteer work in this instance would be contributing to what is an income for the people behind the show and thus considering unethical. Since I don't know any accomplished transcribers myself, I might have (wrongly) assumed it was you wanting to donate your time as opposed to you having a specific skill to offer? I imagine it's not personal and you certainly have a valuable skill to offer that I hope you get other opportunities to channel fulfillingly!