The issue was that previously, DNC officers/etc. pushed preferred candidates behind closed doors - that's what the public had problems with and that's why people resigned. The rule you are referring to was proposed by Ken Martin after David Hogg announced Leaders We Deserve and his intentions to push progressive primary challengers.

Seems like progressives are still getting ratfucked, as usual. David Hogg would've been more powerful internally in the DNC, and honestly, after the DNC ratfucked Bernie, progressives should have been allowed to gain a foothold internally and do whatever they want (within reason) to openly support other progressives, at least for a certain grace period.

It's naive to expect that DNC officers will be truly neutral under this rule. This rule just served to block David Hogg.