He's not allowed to steal real things because it is illegal. He is allowed to steal accents because they aren't tangible things that can really be stolen (the person still has their own accent, after all) and so is perfectly legal. He's a kelpto, so he has to steal something. Telling himself he is stealing accents is how he gets his fix, so to speak.
This reminds me of Brandon's lectures for his writing class, which filmed and poted on writeaboutdragons.com. In the lecture he talks about character flaws vs. handicaps.
A flaw would be something the character needs to overcome, like laziness or arrogance.
Wayne's kleptomania is more of a handicap by Brandon's definition. He doesn't have to overcome it. Instead, he has to cope with it. He probably won't ever stop having the urge to take things, so he "steals" accents and "trades" things.