That is a false statement.
“They did not negotiate or receive permission to observe and inspect from the separatist forces controlling the region they were operating in.”
The separatist forces have no diplomatic recognition, no sovereignty, and no legal authority for their insurrectionist and terrorist activities. Their current status is that of unlawful combatants, insurrectionists, brigands, and pirates subject to capital punishment. Until such time as the separatist forces gain adeqaute diplomatic recognition they remain subject to the laws of the Ukraine or the laws of Russia in the event you assume the separatists are Russian citizens. In either case, Ukraine and Russia are both members of the OSCE, so their citizens are also subject to the obligations of the OSCE Vienna Document 2011. Even if you choose to argue the separatists are no longer citizens of Ukraine or Russia, this just puts you back into the situation where the separatists are outlaws who have forfeited the protections of domestic and international laws. In the case of outlaws, their capture and maltreatment of lawful OSCE inspectors adds to their liability for severe punishment and illegitimacy of the insurrections. If the outlaws chose to reject the OSCE observers, they could have done so without the additional grave breaches of international law incurred by the llegal captures and maltreatment. All the separatists had to do was escort the OSCE observers to the limits of the separatist control and reject the mission. instead the separatists committed very grave violations of many criminal laws.
“If they haven’t received permission, they shouldn’t expect to be treated as if they had.”
First, the only permission required by the OSCE observers was that of the Ukraine, and they had such permission. The United Nations Charter and other international agreements set forth what the separatists can and cannot do with respect to such international peacekeeping missions. if the separatists wanted to violate those laws, they did so at their peril and at the peril of their seeking diplomatic recognition.
Second, even if it was to be assumed the OSCE were not under the protection and immunities of international law, the separatists were obligated to deport the OSCE observers unharmed. Even if you argue the separatists had some kind of nonexistent right to capture the OSCE observers and detain them as enemy prisoners of war, the separatists have committed a variety of grave breaches of international law related to the conventions respecting prisoners of war. The way in which the separatists have subjected the captives to public view while held captive by violent armed force is a grave crime and formal act of terrorism. No matter how you try to argue the case, the maltreatment of the OSCE captives cannot be justified under any form of international law whatsoever.
“If they’re not neutral, and they are not, they shouldn’t expect to be treated as neutral observers.”
It makes no difference whatsoever whether the OSCE observers could be regarded as friendly, neutral, or hostile observers. The laws regarding diplomatic and/or military missions under the auspices of international law obligate every person on the planet to respect the inviolability of the persons in the mission. A sovereign may reject the such a mission and escort the mission out of the sovereign's jurisdiction, but a non-sovereign has no such authority. If the non-sovereign seperatist usurps the authority of a sovereign, the non-sovereign remains subject to the minimum standards which are the obligation of a sovereign.
So, no matter how you cut it, the separatists are in flagrant violation of practically every norm of civilized domestic and international law and human decency.
And those in Kiev who overthrew their legally elected government do?