I'm talking the general case ... not this specific instance with ashes. It doesn't help that TSA from airport to airport can be inconsistent. American (Terminal B) at Logan is pretty good; I've never had problems (knock on wood).
Ashes are not a liquid and you're allowed, for example, to take body powder (i.e. Johnson's Baby Powder) on with no hassles.
And from
http://thehague.usembassy.gov/u2.s._customs.html#ashes:
9. Ashes of Deceased People
You can legally transport ashes of a deceased person into the U.S., if you meet the following conditions:
The ashes should be transported in a non-metallic urn
One should bring an International Death Certificate
One should bring and International Cremation Certificate
The ashes should be carried with your hand luggage
The urn can be subject to inspection, and it is recommended to come early to the airport.
So I would assume same rules apply domestically as well - as long as you have the death certificate ...