- Edited
Śrī Matre Namaḥ,
This is to prevent and discourage people from chanting some mantras which they heard somewhere or read from a book without taking dīkṣā from a Guru. Once a deity shares some esoteric knowledge with a sādhaka, it becomes a responsibility of that sādhaka to guard the mantra and the knowledge of mantra siddhi and transmit only to someone who deserves and if the deity gives permission.
Taking dīkṣā is very important which even deities sometimes remind some Gurus. In one pravachanam by Bhrahmasri Samavedam Shanmukha Sharma garu, he narrated an experience shared with him by some person. This person attended some mediation and lecture sessions on Lalitā Sahasranāmam by some initiated Guru. One day he was so happy, he made everyone recite Śrī Lalitā Triśatī (I think so, I will have to find that clip again) and that night Devī came into his dream and told him not to make any uninitiated person recite Śrī Lalitā Triśatī. The next day that Guru shared his experience with everyone.
And in some cases some mantras may very well be cursed which happen in Tantras because Tantric mantras are the easiest to chant, very potent and deliver results very fast. Unless one has a protection from a Guru, one can do a lot of damage to oneself and people around with Tantric mantras.
Śrī Gurubhyo Namaḥ