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Disc brakes do place more braking loads on spokes than rim brakes can. You can work out what the difference is by taking the ratio of the rim diameter to the rotor diameter.
Force(rim) x d(rim) =force(rotor) *d(rotor)
So for a 160mm rotor a disc brake thd braking torque goes through the hub fglsnge. Braking forces on spokes are therefore 10 times as much load on the spokes as a rim brake can at the limit of tyre adhesion.
cycleclinic-
And all the other braking forces required to turn kinetic energy into heat get magically turned into pixie dust? The mechanical advantage of the braking system varies between disc and rim brakes. The kinetic energy which needs to be transferred into heat does not. Unless your rims are connected to your hubs via magic pixie dust or unicorn hair, all that energy transfer has to take place via the spokes.
Brommers
For the same reason that radial on the drive side rear should not be a thing. The load path is slightly different between disc brakes and rim brakes, but the actual external loads are the same and they get from the tyre contact patch to rider via the spokes either way.
Think about a rim brake for a while; the ground pushes the bottom of the rim upwards and backwards, the caliper pushes the top of the rim backwards (and downwards a bit, as it isn't usually right at the top), and the fork ends push the hub forwards and downwards. All the vectors have to sum to zero at equilibrium. To a first order approximation, the hub is pushed forward by twice the horizontal force at the tyre contact patch and downward by the vertical reaction force at the tyre contact patch. Somehow, all these forces get from the rim to the hub. That somehow is called spokes.