It is interesting to know the reason behind Shiva's boon to Jayadrath that he will be able to hold back all Pandavas except Arjuna for one day.
Jayadrath obtained the boon after failed attempt of kidnapping Draupadi.
But, why Shiva granted the boon to the wicked Jayadrath??
Lord Shiva granted boon to Jayadratha because he has practiced the severest of ascetic austerities as described in Mahabharata ,Drona Parva, Abhimanyu-badha Parva, SECTION XL.
From a keen sense of his humiliation, the king practised the severest of ascetic austerities, desirous of a boon. Restraining his senses from all objects dear to them, bearing hunger, thirst and heat, he reduced his body till his swollen veins became visible. Uttering the eternal words of the Veda, he paid his adoration to the god Mahadeva. That illustrious Deity, always inspired with compassion for his devotees, at last, became kind towards him. Indeed, Hara, appearing in a dream unto the ruler of the Sindhus, addressed him, saying 'Solicit the boon thou desirest. I am gratified with thee, O Jayadratha! What dost thou desire?'
Lord Shiva or for that matter any God is bound to the tapasya. It is respecting that kriya of tapas/observing meditation and lord shiva granted Jayathratha that boon. Jayathratha or any other person has to undergo result of his actions, may it be good deeds or bad. Lord Shiva gave him boon for his tapasya (good deed) and for his bad deeds he was denied from stopping arjuna and he was killed by arjuna.
To another side of the question lord shiva doesnot have any discrimination what so ever it is, he just grants people with what they deserve.Shiva Maha purana explain the nir dwandwa sthiti of Lord shiva.
