That's missing the point; if you sign up to an email list with a stated remit, you're not giving consent to be bombarded with any old crap. Of course WOS needs advertising revenue, but the place for British Gas ads is not an email list which specifically describes itself thus:
"Join our famous mailing list and we'll save you hundreds of pounds a year on your theatregoing"
Opt-in email marketing has specific conditions, and one is that you have to know what you're signing up to.
I find the response from WOS more irritating that the actual email - particularly the idea that we should be impressed that non-theatre ads are a rarity, when according to the terms of the list they should be non-existent.