Baloney. The only obligation a DM has is honesty. Don't cheat, don't lie, and no mid-encounter revisions when things don't go as you expected. It is up to the players to take advantage of any opportunities that occur.
You're not going to have the Right Tools™ every occasion. That's life. You adapt, you adopt. You improvise. The resources are lacking? Get creative instead of kvetching about the unfairness of it all.
And another thing. Not having the Right Tools™ does not mean certain character types etc. can't participate, it only means those characters types have to find another way to participate. The technical term is "Adaptation". If you can't adapt to circumstances, you really shouldn't be out adventuring.