I don't think you're missing much.
Well, maybe about the agents doing editing before submitting. Some do, some don't. Some projects need it, some don't.
But I think you're on to the key thing, which is that the sales aren't necessarily going to be high enough to justify an agent's attention.
If you look at Brenda Hiatt or Veinglory's numbers, a typical expectation from a Samhain book would be $3K-$4K.
A book that makes $4K for the author would, with an agent, make $3400 for the author and $600 for the agent. Over the course of several years. That's not a lot.
I'm a bit believer in agents, too. But when I look at the time my agent is spending on my books (and the time she's spending communicating with me that SHOWS me the time she's spending), I can see how she wouldn't be able to afford to do that for $600 over several years.
There ARE agents who sell to e-publishers, for sure. But MOST agents will submit to the bigger houses first, and then if there are no bites there they'll submit to the smaller houses. They don't walk away from the project entirely, but they resign themselves to an almost-certainly lower commission.
So a reputable agent is not necessarily going to be interested in working with someone who's already skipped over the big fish and gotten an offer from a minnow. I'm not saying they WON'T, but they may not.
There are a few e-first agents out there, agents who apparently submit only to e-first publishers, but there's debate about whether this is a viable business plan for the agent OR the writer. You could check out the thread on the L. Perkins Agency for some discussion on this.
There's nothing WRONG with using an agent for e-pubs, if you're really that insecure about the contract and if you don't mind giving up the 15% AND if you can find a reputable agent willing to do the work for the probably low return. But don't get TOO worked up about the contract. You're a writer. You understand words. You need to put in the time to make sure you understand your own contract, even if you DO have an agent working for you.