Hey Peter,
You can imagine the cnc machine as a mechanical system built between two ends. One end is your milling bit which mills, which is driven and plunged into work material, and the other end is the machine base where the machine’s feet are attached. To be precise the other end is how the workpiece is clamped to the machine base. Now imagine you would grip the one end, the milling bit (please cut power before imagining this!) with one hand, and with the other hand you would grip the machine table where the workpiece is clamped. And now imagine you would try to bend these two ends apart. The milling bit into one direction, perhaps imagined with a tow rope attached to the bit (or the collet) and you heavily pull on it while the table of the cnc machine is pulled into the other direction. This, in an exaggerated way, represents the forces that act on the mechanical system while the cnc does its usual work, i.e. the motors try to drive your bit through your workpiece. If you follow the thought experiment, you would have tried to imagine, where would the machine be bent, which parts are weak points and where do longer levers make the force rise at some local points in the system?
The leverage force means to increase a force applied with a decreased force provided, by using a longer mechanical part. In this picture it is shown that you can lift a heavy object with needing only 1/20th force of its weight, if you use a lever that is 20 times longer (in this case the wooden board shown):
CR, CC BY-SA 3.0, via Wikimedia Commons
A practical example of leverage force is a nut cracker:
Without the nut cracker, cracking the nut is difficult, but with a lever it is easy.
The same way the mechanical parts of the cnc machine are bent by the force that acts by the motors on the workpiece. The parts of your cnc machine may be heavy, and if you watch Ben Myers and his son climbing on top of the Onefinity you may think, no, there is no bending, but of course, this is not true, every system has play. Parts are bending, bearings have clearance, etc. Also the important force that will bend your cnc machine is not someone climbing on top of it, but pulling and pushing on the axle of the milling motor, which I tried to visualize with the tow rope thought experiment. So you may ask yourself, which are the critical parts on my machine? This of course is not to be explained as quickly, as it is a very comlex system, but of course the critical points can be where leverage force is high. I think this should be treated in a dedicated publication, and only after precise measurements. But you could say in general, if you install the Riser Blocks, you make the leverage force higher with which all the forces that already where applied to bit, collet, axle, router bearings, router mount, Z linear bushings, Z rails, Z slider, X gantry block, X linear bushings, X rails (upper and lower, possibly in different directions because of Z assembly pivot), and finally X Rail feet, now are exerted on the rest: Y gantrys, Y linear bushings, Y rails, Y feet, machine base, workpiece clamps, workpiece. Of course the question is: but how much, is it relevant? As a deeply scientifically thinking person, I can only say, this has to be studied, but of course experience can lead to make a few presumptions, and in this regard I would avoid making the X Rails feet longer, except if I can distribute the increase in leverage force over a larger area, e.g. by a longer Y gantry with double number of bearings. But, to be clear, I have not tested it with your Riser Blocks, I can not provide numbers, this would have to be tested with a before-after-measurement. I can only say, they will add at least a minimal amount of play and minimally decrease rigidity and stiffness just by increasing the lever.
You asked for the screws, I would think they are part of the whole system. Of course the force would be towards the bolts but the force is towards all the parts in the chain between milling bit and table/workpiece. In the first place it’s making the X feet longer which adds leverage force. This does not mean that the riser blocks have to be the weakest point then.
PS: Regarding weak points with high leverage force see also here.