Dear Babak what you have written is simply wrong. If Arab leaders cannot oppose Khamenei’s comments publicly it is for solely political reasons i.e. the Arab street would go crazy and such leaders would be stripped of all credibility in the eyes of their own publics, since they would be perceived as American and Israeli stooges – an image they are always endeavoring to shed.
First of all, Khamenei’s religious credentials are a total joke – the senior Shi’a clergy have little to no respect for him – one only has to look at the criticisms of Khamenei made by Ayatollahs Sanei, Azari-Qom, Shirazi and Montazeri to see that he has little credibility at a purely religious level – or what in the Shia world is called marja’yyat. The concept of an Ayatollah al-Uzma, or an religious scholar who is primus inter pares was itself a 18th century creation and Ayatollah Boroujerdi was the last individual to be regarded as such and he died in 1961. Even Khomeini wasn’t regarded as such and he was in fact criticized at the peak of his powers by figures such as Ayatollahs Shariatmadari and Taleqani.
The Association of Seminary Teachers in Qom only recognized Khamenei as an Ayatollah in the early nineties because the Revolutionary Guards surrounded their offices and threatened to storm them if they refused to relent. As I’m sure you know Khamenei was a Hojjat-el Islam and "promoted" to the rank of Ayatollah in 1989 after Montazeri was pushed aside for criticizing the 1988 prison massacres of MEK members.
Second, the notion of a Faqih and especially velayat-e faqih is very particular to Khomeini and certain authoritarian strands of Shi’a thought. No living marja’ al-taqlid accepts the doctrine of velayat-e faqih i.e. the declarations of an Islamic Jurist are equivalent to divine edicts here on earth – in fact Khomeini said that the edicts of the Faqih even override the Sacred Law i.e. Sharia, if it is in the interests of the state – evidence of Khomeini’s cognizance of Realpolitik – putting reason d’etat ahead of religious law. Finally, the very notion of a clerical hierarchy that issues edicts to be obeyed by a populace over which they rule is a unique situation and particular to Iran‘s Islamic revolution and Khomeinist doctrine, but can also be said to have had its precursors in 19th century Qajar Iran e.g. the Tobacco boycott of 1891-92 etc…
The more non-political version, where a Shia can choose to abide by the edicts if he so chooses of a noted mujtahid refers predominantly to one’s ethical and practical conduct (but also issues of worship) – which again is a completely Shia notion without any power to bind even the followers of the particular mujtahid, let alone Shia who follow another mujtahid (e.g. Sistani, whose religious credentials dwarf Khamenei’s) let alone non-Shias. Sunni thought and religious organization really have no equivalent in terms of a hierarchically structured clerical class who can (contentiously) claim a right to wield worldly power on religious grounds (i.e. as the best equipped to interpret religious law), as in the Shia case in the absence of the Mahdi, the 12th and final Imam who remains in ‘occultation’ until the end times. This is why almost all Sunni radical groups have been mobilized and initiated by laymen – from Qubt to al-Zawahiri.