I also sent dust transactions once or twice, with no fee added, and they went through. And surprisingly, they didn't even take very long. A couple of hours, maybe.
But that was already some time ago, could very well be that the Bitcoin core was changed to prioritize such transactions differently, in the meantime.
Its a rule of thumb based on the required priority for a transaction to be allowed in the 50KByte reserved for TX without fee. Obviously any miner can change these default settings as they deem fit. E.g. a TX that creates or spends dust (<5460 satoshi) can be accepted and mined by, but will not be relayed by any node running the default config. Thus its hard for such a transaction to reach a miner in the first place.