This is because of the roles of Trideva.
Hero doesn't stop director or script writer from making villain a powerful character that would create chaotic situations in film.
What does that mean?
Let me explain it by simplifying it and explaining it in layman's language.
I really enjoy explaining about the world and Sanatan Dharma by simplifying it as a movie. Even Niralamba Upanishad states that this world is not real, the only Reality is God (ब्रह्म सत्यं जगत मिथ्या). One Eternal Consciousness is Real, otherwise Maya that takes us away from non-duality and keep us attach to this world.
Let us consider the world as a film. We need to understand the roles of Brahma, Vishnu and Shiva. We have 33 Koti Devi Devtas and each of them have different roles though there is one Ultimate Consciousness Para Brahman that has these many forms.
So Brahma is the creator, which means he is the script writer ✍️, He is Vidhata and he is the dispenser of the destiny of all the characters including positive and negative.
Now since the writer has to prepare a fair script, giving the scope to all characters, Brahma does give the boons to Asuras in that sense.
Then we have the Preserver, Vishnu. He is Bhagwan, the central character, the lead hero of the movie.
यदा यदा हि धर्मस्य ग्लानिर्भवति भारत।
अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम्॥ (4.7)
परित्राणाय साधूनां विनाशाय च दुष्कृताम्।
धर्मसंस्थापनार्थाय सम्भवामि युगे युगे॥
Whenever Adharma dominates, He incarnates in various forms, defeats the Asuras and establishes Dharma. This is the exact work of a hero, Vishnu as Varaha slays Hiranyaksha.
As Narsimha he slays Hiranyakshap.
As Parshuram he slays Sahasra Bahu.
As Shri Ram, he slayed Ravan, Kumbhakaran and others.
As Shri Krishna he slayed Kansa, Shishupal and many others.
And in his last incarnation as Kalki, he will again defeat the evil and restore the order of the world.
So Vishnu is the main hero while Indra and other Devtas are the supporting characters. You will never see Vishnu giving boons to Asuras because he is the hero who has to defeat the villains.
Now let's come to Shiva. He has an entirely different role although. He is Ishwar. He is the main director of the film who has the authority of running this world in the way he feels like.
A director is also like hero but in a different way. Hero has to directly fight with evil characters and defeat them. So Vishnu enters the system and brings order and balance. But Shiva is more transcendental in a sense that he remains out of the system, he remains in Samadhi but has his third eye open to see the world, just like the director monitors every scene from behind the scenes.
A director does not differentiate between hero and villain when it comes to providing them the roles. He has to provide role to every character, so in the film, villain also has the scenes where he is dominating the good characters, just like Asuras after getting boons would defeat Devtas and capture Swarg Lok. So everyone worships Shiva, be it Devtas, Asuras, Rishis, Munis, or all others. Every character, either positive or negative has to maintain good relationship with the director.
So it's not a wrong thing done by Shiva or even Brahma. This is the role of the main director and also of the script writer/casting director to assemble the characters in film and give them their powerful roles.
So Mahadev is the one who can even change the script written by Brahma, like if director wants to make changes in script, he can definitely do that. Just like Rishi Markandeya was supposed to live only for 16 years, but since he being a positive character showed his potential and was a great devotee of Mahakal, he got the boon from Shiva and was saved by him from Yamraj. Similarly negative characters also got some or the other boons depending on their potential and Tapasya they did to impress the director Shiva.
So Vishnu doesn't stop Brahma or Shiva from giving boons to Asuras because none of them are wrong. They are all knowing and do their work.
Since the hero is the dearest character of the director and in the end of the film, the director would definitely support hero only. Hero is the exact projection of director's idea about the film and perhaps hero is the one who makes director's film hit and hero also needs director to thrive in his role. Director and hero complete each other.
So Narayan is the dearest of Shiva and that's why they always support each other.
So I hope my answer would help in understanding Sanatan Dharma better with the help of cinematic perspective.
Jai Brahma 🙏Jai Vishnu🙏Jai Mahesh 🙏