A US law prevents the government from selling arms to countries which fail to tackle human rights abuses.
Mr Buhari met President Barack Obama in Washington on Monday to seek further assistance.
Boko Haram has killed some 10,000 people since 2009 and has also kidnapped hundreds of girls and women.
Last month, human rights group Amnesty International said that some 8,000 men and boys had died in Nigerian military custody after being detained as suspected militants.
The military rejected this allegation, Nigeria's president promised an investigation but there have been no further details.
The US has previously promised some $5m (Â£3.2m) in military assistance to the regional coalition helping to fight Boko Haram, which has pledged allegiance to Islamic State.
However, Mr Buhari heavily criticised the US, especially the Leahy Law, which links military sales to human rights.
The Nigerian military did "not possess the appropriate weapons and technology which we could have had if the so-called human rights violations had not been an obstacle," he told the United States Institute of Peace on Wednesday.
"Unwittingly, and I dare say unintentionally, the application of the Leahy Law Amendment by the United States government has aided and abetted the Boko Haram terrorists."
In the latest suspected Boko Haram attack, at least 29 people were killed in bomb blasts at two bus stations in the north-eastern city of Gombe on Wednesday.
At least 11 people were earlier killed in the Cameroonian town of Maroua, just over the border from Gombe.
Boko Haram last year seized a huge area of north-eastern Nigeria, before being beaten back by a regional coalition, including Cameroon, Chad and Niger.
Mr Buhari was elected in March, partly on a pledge to defeat Boko Haram.
Why Boko Haram remains a threat
Who are Boko Haram?
The US has "aided and abetted" the Boko Haram Islamist militant group by refusing to provide weapons to Nigeria, President Muhammadu Buhari says.