Minakshi, very good topic 👍🏼
Actually, Brahma had a history of doing this. Most famous example being Ravan, but there were others - Hiranyakashipu, Mahishashur Mardini, Indrajit and some others I can't recall. End result being that either Vishnu had to take an avatar, or sometimes, either Vishnu or Shiva had to do some strange things. For instance, Mahishashur's sister Mahishi wanted a boon that she be killed by a child born to Shiva and Vishnu, thinking that it was impossible. That's why Vishnu had to take the form of Mohini and give birth to Ayappa to do it.
Shiva too did it on occasion - he granted Surapadman a boon that he won't be killed by any power except him. End result was that since Shiva was unmarried (after Sati's death), the devas tried to get him to marry Parvati and get a son who would put them out of their misery. That son was Kartikeya. He granted Madhu a boon that as long as he or his son had his trishul, nobody could kill him. So Shatrughan had to catch Lavanasura unawares w/o his trishul. Shiva also granted Banasura the boon that he'd not be defeated by anybody other than one equal in power to him.
Anyway, back to your question: I partly agree w/ you that Brahma is responsible, but remember, the original boons requested from all these demons were immortality, whereas the boons ultimately granted were watered down versions w/ loopholes that would allow Narayan or Mahadev to kill them under those conditions. With the 3 gods, if someone did years of tapas, it would have been unlawful of them to not grant them anything of their choice. Even the devas on occasion appealed to Brahma not to grant them what they wanted, but that was a non-starter.
Point is once these demons got their power, they became arrogant. Most of them (that we've read of) did abuse their power, like the ones listed above. But a few didn't - Bali, the grandson of Prahlad, did his penances only to avenge his father, while Vibhishan only sought to be righteous, and despite being granted immortality that no-one else, not even Ravan got, he never persecuted anyone. But the ones who did, their crimes had to reach a certain threshold b4 Vishnu or Shiva started acting: none of them could preemptively punish anyone w/o looking unjust.
Curiously, I'm surprised that none of the demons ever meditated to Vishnu asking him for such boons. In fact, the only one I recall meditating to Vishnu was Dhruva, and that too for the simple right of sitting on his father's lap: Vishnu granted him his father's succession (despite him being younger) and a constellation named after him. Other than that, nobody ever approached Vishnu for such a boon.
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