Okay I should have said almost equal, I agree with you.
But this does not change the fact, that IS-2 costs 50 fuel more, requires 12 CP and is present in only 2 commanders.
At the same time Tiger and IS-2 cost the same but the fights are one sided. Also the tiger is present in more doctrines and they can be very versitile.
The Panther is an AT unit, the IS2 is not. That's why it costs extra.
From what I know the fights between Tiger and IS2 favor the Tiger. However it is not the only thing to consider. Allies pay less to get a reliable counter. If Allies go for heavy tanks, Axis is almost forced into a Panther (I know JP4/Stug CAN work, but they are not reliable).
I don't think that the balance between Panther and Allied heavies is far off. The Panther usually cannot kill them in a 1v1, but it is good enough to block any kind of break through attempts and punish/kill them if they have been mispositioned.
If you think the Panther should not be sufficient to retain an IS2, why should Axis be forced to invest more resources and pop into counters that can be used for AT only (you previously talked about Panther + StuG/JP4, so about 30 pop), while the Soviet player should only invest 21 pop into an allrounder? That would be actual misbalance.
As Axis you currently invest less resources and pop into a unit that will be mostly busy with pushing back vs the IS2. If you manage to cancel the IS2's effectiveness (which the Panther definitely can), you've gained a small pop and resource advantage to invest somewhere else. If not, the Soviet player can rightfully gain an advantage which is fair because his investment is higher.
And to make it clear: I am not saying that everything about the IS2/heavies in general is perfect, but overall the matchup Panther/Allied heavy is not far from the optimum.