Who grants ebike-ready certifications for tires (aside from manufacturers)?
For the name brand tires that are certified 50km/hr there is an actual international standard that the tire manufacturer is certifying against.  These are tires that are built to the UNECE motor vehicle tire specifications instead of bicycle specifications.  

For example all Schwalbe tires (at least for 2019 product line) that bear their  "Energizer" badge printed on the sidewall are certified to that standard.

For tires that are recommended for 25km/hr it appears to be purely manufacturer's recommendation.