Has there been any explanation why the requirement to have it move thru the fairlead still exists ? Is it to save ourselves from having to spend $10 on a block ?
Possibly because this is a fundamental rule or possibly because no one has suggested a rule change. I am pretty sure it would have been covered when the new control systems were developed.
It's not a "fundamental" rule, just a regular one that happens to be written that way, for whatever reason. It's kind of similar to the rule that requires the cunningham line to move through the grommet.
Someone at my club had rigged his/her outhaul in a similar way a couple of months ago, I guess people just come up with this independently of one another. I remember thinking then, "now that's illegal"!
It's likely the rules were written to prevent the use of the block being attached to the fairlead because a block on the fairlead and on the sail cringle would hit each other and not be fully adjustable when using an older stretched sail. The ILCA technical committee tries their best to make sure if you adopt a change that you only need to change the part being changed by the rule change, and not multiple other bits of equipment.