There's 2 reasons for cursed ring being lower.
1: The cursed ring is made for AoE damage. It should not have higher single target damage output than the ring that was made for single target damage output.
2: The cursed ring is more of a defensive ring compared to the other two, it is more intended to be a tanky presence with decent AoE damage. Hence why the HP bonus on this ring is higher than on the others.
To respond to a few other points in this topic:
As for the infinite kick thing. I personally feel like it should remain the way it is. It's what makes TKs viable, without it there would be no reason to really be a TK over a star glad. Star glads do their single way higher dps kicks at the right stance, TKs should do lower damage kicks but able to trigger and combo them faster. Star glads give up lots of bonusses to have the higher burst damage as well. The delay you mention is always an option if the spam turns out to be too strong. However in that case I would always prefer lowering damage over reducing the spam of kicks as the continuous spam is what makes a TK ranker stand out from Star glads.
Lastly I see people comparing TKs with SinX. This is a complete misinformed comparison. Taekwon rankers were implemented by Gravity as a constant moderate damage dealer and crowd controller with great tanking/survival capabilities. They were never made to outdamage a sinx. Sinx was implemented to be one of the highest burst damage classes with little survivability other than their agile movements. The way it is right now (after the change) sinx need to use 2 weapons to outdamage the TK. with that they risk getting more damage too. To me that's the way it should be. If you want full out damage you can go Sinx, if you prefer an all around and more safe option TK ranker now is a viable option.