I'm not sure the law can be bothered to delineate the difference. It's the intent of the service provider rather than the needs of the users that they'll go after.
I don't deny that authorities can probably spin facts if they're motivated enough (even if mixers aren't inherently illegal), but at least in Bestmixer's case:
A mixing service will cut up a sum of Bitcoins into hundreds of smaller transactions and mixes different transactions from other sources for obfuscation and will pump out the input amount, minus a fee, to a certain output address. Mixing Bitcoins that are obtained legally is not a crime but, other than the mathematical exercise, there no real benefit to it.
The legality changes when a mixing service advertises itself as a success method to avoid various anti-money laundering policies via anonymity. This is actively offering a money laundering service.
It looks like this case also advertised itself as a way to launder money, which made it blatantly illegal on top of everything else. I don't think other mixers should be too concerned just because a random one got taken down.