"They" might say the intention of bestowing wealth on others is one incentive for accumulating it, meaning saving and investing. Hence an obstacle to bequests, such as a tax, is a disincentive to save and invest. You are right that there is no symmetric argument from the point of view of the heir. There is no argument I can think of to justify the untaxed receipt of the gift or bequest.
A more stark sort of dissonance is that inherent in the different ways alimony, child support, and public assistance are regarded. Privately financed alimony or child support never raises questions about the character of the recipients. Only publicly- financed transfers do.
For a proposal to socialize inheritance (at least partly), take a look at The Stakeholder Society, by Ackerman and Alstott.
mbs