Many of the asurAs worship Lord shiva and get boons as observed in the purANAs. Whereas very few of them worship the other divines. Why is it so ? Why should the God show grace to asurAs ?
Lord shiva is the Supreme, Which does not have any parents Which never takes birth Which is all alone without association with any of the creatures or creations enjoying in the Self. God is the only one Who is dependable for anybody / anything to surrender to as It is the only perennial Being. Hence God is the Lord of all creatures (lives/souls). For this reason the Lord is hailed as pashupati (Lord of living beings). Whether it is devas or asuras or humans or other creatures all are pashus. That being the case how could the Lord be partial to one section of pashus and withdrawing the Grace for the other ? So anybody who worships the Lord sincerely could get blessed with Its Grace irrespective of the caste, creed, race, power, status and qualities.
It is in fact to be noted that Lord shiva is worshipped by the devas like viShNu, braHma, indra, by asuras like bANa, rAvaNa, tripura, sUrapadma, by humans like sha~Nkara bhagavatpAda, samban^dhar, appar, by other creatures like jaTAyu, sampAdi (eagles), vAli (monkey) and the list goes on and on. One finds in the purANas the variety of people of different backgrounds and qualities worship the Lord shiva. There are many histories of temples which talk of the cranes, bees, elephants, spiders, snakes worshipping the Lord and getting blessed. So the Lord as the Supreme blesses anyone who worships in sincere devotion. There is no discrimination on who the seeker is. The Lord is so merciful that He showers the boons one look for when there is a determination to seek Him. It is evident from the history of asura bANa who attained a great fame of his valor and got the place in the abode of Lord shiva all due to his determined worship of the Lord.
But..., if the receiver of the boons does not use them in proper way and misuses to disturb others, in the Lord defined system of this universe they get punished for the misdeeds. A closer analysis would show that the devas are the divine elements of nature that are essential for all of us to survive (like air, sun, fire etc). Very often asuras are the one who disturb the nature as evident from purANas. So when the asuras misuse the boons got to disturb the other lives they get punished as evident from the histories of hiraNya, rAvaNa and others. When the misuse goes beyond a point by the powerful forces so as to question the existence of the whole system, the Lord the One Who is essential for all the essentials, Who has devised this whole Universe, takes the action to restore normalcy for example like tripura dahana.
Lord shiva is the God of all. Like the mother He showers the grace for all the children, but the misusing children get punished. This Supreme Lord better than a mother does not withhold the grace, He is our beloved pashupati.