Air carrier jumpseating Captain reported the flight crew misinterpreted ATC's nonstandard clearance of speed at your discretion" and accelerated above 250 KIAS below 10;000 feet."

Narrative

I was jumpseating SFO - ZZZ. During climb below 5;000 ft.; NorCal Departure cleared the pilots speed at your discretion." I observed the Captain tell the FO to delete the 250/10000 line on VNAV climb page. Then he started to accelerate. I was sitting in the second jumpseat so I asked the FO why we were accelerating above 250 KIAS below 10;000 ft. He said that ATC had given them high speed. I told him that we are never authorized to exceed 250 KIAS or clean maneuvering (if higher) in the US airspace. The FO did not correct the Captain; so I spoke up and advised the Captain not to exceed 250 below 10;000 ft. He too told me that ATC had given us high speed. I corrected him as well. We did have a thorough discussion of the applicable FAR 91.117(a).This is a regular occurrence climbing out of SFO as NorCal is in the dangerous habit of giving the nonstandard clearance of "speed at your discretion." This has been occurring for years and I observe it regularly giving line checks and on line flights where the pilots mistakenly interpret that instruction as a clearance for high speed below 10;000 ft. Several of us Line Check Pilots (LCPs) have been reporting this and have called NorCal TRACON to try to get them to stop this practice. We have had no effect on stopping this from happening."