Congrats on getting published.
I work in theatre, and have noticed a growing (and infuriating) trend in people trying to blag services and equipment for nothing, asking for people to work for 'expenses and experience' as they're providing an 'opportunity' that's "good for the CV"... All it's good for on the CV is showing you will work for no money, undervaluing your skills/time/experience and devaluing the industry as a whole. I also write freelance, and have sold a couple of pantomimes.
I agree with those above who advise seeking payment for your submissions - especially if your work will be sold on - as you have obviously invested your time, creativity, and effort in your work, and that is undoubtedly worth something. If they disagree, as has been said, they are ripping you off or ripping off their customers.
Now maybe, reading your posts, you feel there has been a trade-off in this instance - certainly, if you feel that you have received advice/feedback etc that has been invaluable - and that's fair enough. As long as you are getting SOMETHING for your work, and are happy with the situation, then it's no-one else's business. If they continue to use your ideas, though, or if you submit ideas/articles to others, then I don't think it's unfair to expect remuneration.
That's my two cents, consider it chipped in
