Errm, the lopsidedness is in the Klaestrons' favor -- they can extradite whomever they want (in this case, Dax) from Federation territory without needing UFP permission. Why would the UFP have forced such a treaty on them? If anything, it suggests the UFP was bending over backward to accommodate the Klaestrons. Which raises the question of why they'd do that.
Well, one would think that even if the Klaestron weren't UFP members, the Feds would desperately want
them to be. After all, they seemed to be living in the space near Bajor, space that probably was hotly contested in the recent Cardassian War. And basically everybody in that space seems to be less than enamored with the Federation - the Miradorn, the Kressari, the Xepolites, and now the Klaestron. It would have been nice to have even one
sympathetic shoulder to lean on there...
We should note that the treaty is not said to be one-sided. Yes, it allows for unilateral extradition - but probably by either party. That is, one side can take what it wants, and so can the other
, without consulting the counterpart. That sounds like expedient frontier justice, which is what the Federation would be dealing with on this former battleground. Before there is mutually compatible law, there is mutual acceptance that the laws of the other should be respected. And the supposed crime of Curzon Dax was indeed committed under Klaestron jurisdiction, even though Dax subsequently fled to Federation space (and then unwisely loitered to neutral space where s/he could be captured).