Preponderance of the evidence is the standard burden of proof for civil cases, whatever actors are involved -- the side the wins is the side that proves that its position is most likely to be correct.
The higher (beyond a reasonable doubt) burden of proof in criminal cases exists as a safeguard because of the greater consequences to liberty of a criminal conviction.
Its the same standard used when pretty much anyone wants to use the legal system to take your money or other property via a lawsuit, so I don't think its that outrageous that it is used for civil forfeiture cases.
If we're talking about the police alleging that you have money because of criminal acts, we should switch to the criminal standard.
So, the principle on which the existing distinction is based really doesn't support it being applied to forfeiture actions.
And, while there might be a good argument for changing the principle underlying the distinction, I don't see a clear reason for applying specifically the beyond a reasonable doubt standard to forfeiture (or even a higher standard that would apply in any other civil claim.)
It's hard to convict you of a crime for driving around with a envelope stuffed with a lot of cash. But's it's easy to demonstrate reasons why the cash may be used for nefarious purposes.