gwraigty
Thinks s/he gets paid by the post
An educated guess on my part is that since if they did offer beneficiary designations for joint accounts, the only situation where the beneficiaries would come into play is where the joint account owners died simultaneously... if one died before the other then the ownership of the joint account would go to the surviving joint account owner (who could then change beneficiaries if they wish to).
Since whether the joint owners died simultaneously or not can be subjective, by not allowing beneficiary designations for joint accounts they avoid the simultaneous death issue entirely and the possibility of paying out to the beneficiaries based on a simultaneous death that is later changed and results in a change in how the proceeds would have been distributed.
For example, say Dick and Jane die in a car accident, the deaths are judged to be simultaneous and the proceeds are distributed to 5 people equally. However, a case can be made that Jane died first and Dick's will names his son Joe as sole beneficiary. After the joint account proceeds are distributed, Joe files suit claiming that Jane died first so when Dick died he was the sole owner of the account and that Joe is entitled to the entire proceeds as Joe is Dick's sole beneficiary in Dick's will.
If Joe wins the case in court and the court decides that Jane died first and that Joe is entitled to the entire proceeds then Vanguard would have to find a way to clawback the distributions (which may have already been spent) or be on the hook for the difference.
Then why do other financial institutions allow beneficiary designations on joint accounts? Their legal departments are much more aware of these possibilities than the average person.
Joint accounts WROS usually aren't governed under a will. States also have varying laws about survivorship, usually measured in days. Then there's the Uniform Simultaneous Death Act (120 hours), which may govern when there is no will, or a beneficiary of a life insurance policy and the insured die at the same time. In the latter case, the named alternate beneficiary would get the policy proceeds.
My state has a Simultaneous Death Act that says (in the absence of a longer survivorship clause in a will) that unless there's clear and convincing evidence that the survivor of jointly owned property WROS survived at least 120 hours after the death of the other (assume only 2 joint owners for simplicity), then ownership reverts to joint tenants in common. 50% of the property would then pass via probate under the terms of each decedent's will. POD accounts/property are specifically exempt from probate.
The above leads me to believe that Joe would receive proceeds of the POD account only if he is a named beneficiary, and in the percentage designated. POD designations always trump what the will says, or intestate law.
And I've done enough online research on this matter for today. It's easily confirmed with a little effort at your leisure.