There was no one willing to accept the body and, the precedence has been set with the case of Osama bin Laden.
Hendawi Hamza, a professor of Islamic Law at the University of Jordan stated, "burying at sea was permitted if there was nobody to receive the body and provide a Muslim burial…” Hamza, H. (2011)
Floyd Thomas, the chairman of Caroline County's board of supervisors where tsarnaev is said to be buried commented, "I think we'd try to undo what's been done...” Lewis, B. & Lavoie, D. (2013) as cited on yahoo new..
Ref:
Hamaza, H. (2011). "Islamic scholars criticize bin Laden's sea burial", San Diego Union Tribune,
Retrieved May 4, 2011.
Lewis, B. and Lavoie,D. (2013), Boston bombing suspect buried in Virginia cemetery, Associated Press