Brahma Loka and Vishnu Loka are one and same. Shiva Loka refers to Kailash, Shivpuri and Mansarovar Lake. The reason why Shiva Loka is not Brahma Loka because Shiva has to stay on earth so as to end the world at the end of the Kalpa. He then goes back to the cosmic ocean and then comes back on earth in the Satyuga. But that doesn't mean that Shiva is lower than Narayan.
During the Narsingh Avataar period, when Shri Narsingh became very furious after killing the demon Hiranyakashyap, No one was able to pacify him. Lord Shiva manifested as the most Ugra and fearsome God of Hinduism, Sharabheshwara and then attacked Lord Narsingh and made Narsingh realize that he is Shri Vishnu. Here, Narsingh praised Sharabheshwara and only then did Sharabheshwara let him go free. Even after this, Lord Shiva himself said that even though Sharabheshwara Avatar won over Narasimha Avatar, Shiva and Narayan are one and the same and no one should distinguish between them or they will rot in hell. The same was said by Vishnu on numerous occasions.
The reason why Vishnu is worshiped for going to Brahma Lok is because he and Brahma are the ones from whom every living being including humans manifested. They only can give you the final Moksha. Shiva on the other hand is a Kalyaankari Shakti and hence he has the power the give long life or immortality. Lord Vishnu or Narayan usually never gives the blessing of immortality because he has the Moksh giving power and Moksh is achieved when one is free from sins.
For gettiing final Moksh one has to worship only Brahma or Vishnu. But since Brahma has been cursed by Shiva, he cannot be worshiped. But just because Vishnu has the Moksh giving power doesn't mean Shiva is less powerful. Both are one and the same. They both finally merge with the eternal Sadashiva.
Unfortunately, the kind of stories being shared by ISKCON in the name of God are very shameful. And what is even worse is that the Indian crowd in general is blindly following their preaching without even asking the question "why" and "how". As Vishnu always says, those who distinguish between me and Shiva are bound to suffer in the Narak Loka.
Disrespecting Shiva or Vishnu for any reason is a very grave sin in Sanatana Dharma. As a result of disrespecting Shiva, Lord Vishnu himself will punish those who preach falsehoods about Shiva and send them to hell.
One really great example about ISKCON's wrong doings is that it has become full of corrupt and egoistic Gurus and authorities. There is unbelievable amount of dishonesty, egoism, pride, hatred, back talking, abuse of power, zero ethics, hatred and downgrading women, careless attitude of gurus towards their disciples and so on. They have proved to be nothing but shame for Sanatana Dharma. After seeing the repeated blasphemy spread by ISKCON, it became very necessary to spread this knowledge. The preaching of ISKCON should be thrown away from one's minds if one does not want to go hell after death.
This article is an continuation of the earlier article on the powers of Shiva and Vishnu and which of the two is the superior God. That article can be seen here - Shiva and Vishnu