It is the nature of the material qualities that when one becomes very opulent in terms of wealth and prestige, one loses one's sense of etiquette and doesn't care about anyone, even a sage like Narada Muni. For such bewildered persons, who especially deride devotees, the proper punishment is to be again stricken with poverty. A poor man can be easily convinced that the prestige ofan opulent position in this material world is temporary, but a rich man cannot. 

Therefore Narada Muni set an example bv cursing Nalakuvara and Manigriva to become trees. This was a fit punishment. But because Krishna is always merciful, even though they were punished they were fortunate enough to see the Supreme Personality of Godhead face to face. Therefore, the punishment given by Vaisnavas is not at all punishment; rather, it is another kind ofmercy. By the curse of the devarsi, Nalakuvara and Manigriva became twin arjuna trees and remained in the courtyard of mother Yasoda and Nanda Maharaja, waiting for the opportunity co see Krsna directly. 

Lord Krsna, by the desire of His devotee, uprooted these yamala-arjuna trees, and when Nalakuvara and Manigriva were thus delivered by Krishna after one hundred years of the devas, their pure consciousness was revived and they offered Krishna prayers suitable to be offered by demigods. Having thus gotten the opportunity to see Krishna face co face, they understood how merciful Narada Muni was, and therefore they expressed their indebtedness to him and thanked him. Then, after circumambulating the Supreme Personality of Godhead, Krishna, they departed for their respective abodes.