A possible ameliorating factor: they had an IAS Disagree warning (unreliable airspeed) since takeoff, and it may be policy to keep airspeed up in that case -- n

A possible ameliorating factor: they had an IAS Disagree warning (unreliable airspeed) since takeoff, and it may be policy to keep airspeed up in that case -- not knowing your airspeed means that reducing it may cause a stall. (Still, they could have asked ATC for an speed reading or used other measurements to convince themselves that it was safe to slow down.)