I'm actually quite interested in ownership of natural resources and different liscening systems (hey, it gets boring offshore OK?) and the US is unique in the fact that oil and mineral resources are owned by the particular landowner. The landonwer can sell some or all of the mineral rights to an oil company (or set up their own oil company).
Everywhere else these resources are owned by the state. There are then various way that the resources can be exploited:
1. The state may then give (or sell) a licence for a particular area to an oil company, who then owns the natural resources for the duration of the licence outright. This is the situation in the UK, Norway and Canada amongst others. The oil company pays corporation taxes and royalties on the produced oil. The tax and royalty rates can change: Gordon Brown, the UK Treasury Secretary has increased taxes on UK oil operators three times in the last few years for example.
2. The state may keep ownership of the oil and the exploitation is done by a National Oil Company, that is effectively part of the government. This is the situation in Saudi, Kuwait and Iran. This was the situation in Venuzuala and Brazil too, but they then moved to PSAs (see below).
4. The state may keep ownership of the oil and the exploitation is done by an oil company who is paid for their services. This isn't popular with oil companies who can't book the oil reserves as their assets. This is the situation in Iran.
3. The state signs Production Sharing Agreements with an oil company. These are a fudge: the state keeps ownership of the oil but the oil company "owns" it too (and can book the oil reserves on their asset sheets). The oil company is usually the junior partner with a National Oil Company, and is often the operator. The oil company's capital and operating costs are paid out of the oil produced ('cost oil', it's called, and so the infrastructure isn't theirs) and takes a share of the remaining 'profit' oil. The oil company also pays any taxes and royalties on their oil. These tax, and royalty rates may change: that is what has just happened in Venuzuala: the oil companies were told that their royalty rates would increase to 50%, and that corporation tax would also increase)
The Putin govenment feels that the PSA signed for Sakhalin I & II were weighted against the interests of the Russian government and has been putting pressure on Shell to allow Gazprom to enter the PSA. This pressure has included environmental complaints, and accusations of incompetance regarding the cost escalation. Shell will be reimbursed it's costs to date for reducing it's interests in Sakhalin II, (just as the US oil companies in Aramco were reimbursed when the Saudi's bought it to create Saudi Armaco, the Saudi National Oil Company) but that's not really a consolation for all the lost future production and reserves, or the loss of control at becoming the junior partner (and maybe losing the operatorship?).
Another issue involved with Russian oil is that the privatisation by Yeltsin of the Russian oil industry in the 1990's was massively corrupt: no-one like Abramovitch or Khodorkovsky paid anything like market prices for Yukos or Sibneft, but they were well placed pals of the old regime, and became billionares overnight. Putin is trying to wrest back what he (and a lot of the Russian people) think was stolen from them. His tactics involve false and trumped up charges and other pretty nasty things, though.
Apart from outright appropriation of an oil company's assets (which has happened in a couple of countries in the past) any government is perfectly allowed to change tax and production royalty rates- that is, after all, what governments are there for!!! International law doesn't come into it. An oil company looks at how stable the fiscal regime is when it decides to invest in a country- for example, there are rumbles that the UK is becoming a fiscally unstable place- along with the geological risks and the technical risks.
As for Russia, I don't think there will be anything like this from now on: future PSAs will obvously be signed on less favourable terms to the oil companies, who will base their economic models on different numbers before deciding to go ahead with any projects in Russia.