It sounds to me like lots of Dropbox customers opened a single Adwords account each to run Dropbox ads, and Google mistakenly concluded that they were all the same person.

I don't think Google has any illusions that it's the same person. Instead they see it as the same beneficiary.

If this weren't a problem we could have a Hacker News "free ad" board (with "free" meaning "at the cost of other advertisers". Due to the bid system free ad coupons cost Google next to nothing) where people could post their pet site and hundreds of people could use some or all of their "free" $100 to yield tens of thousands in free ads.

Not exactly. If you read his post closely, he claims to have only had a single account. It's Google which is claiming that he "maintains multiple AdWords accounts simultaneously serving ads for similar or related keywords and/or businesses."

This makes me wonder if we are hearing the whole story. Did he have multiple adwords accounts, or only just the one?

I only had one adwords account.

This can't possibly be a violation -- this is standard operating procedure in the SEM world.

One campaign, multiple ads. Which as the other commenter says, is totally the norm.