Lord Shiva had given many boons to many Asuras although the Asuras are not good. Why doesn't Lord Shiva hate the Asuras?
If someone does penance,whoever he might be, whether a Deva ,asura,pisaca or a manava, he deservedly earned his rights to boons.So, the God is bound to give him boons within the permissible limits,and it does not matter at all for what purpose the devotee is asking for that boon or what he does with that boon in future.
So,in short,it will be against Vidhi ke Bidhan(the divine system of laws) for any God to deny a devotee his boons once he had done sufficient penance.
Asuras not only do penance to Lord Shiva but, also (as seen on numerous occasions), to Lord Brahma as well.
Lord Vishnu ,who being the preserver of Dharma, is apparently and naturally somewhat inimical towards the Asuras who are always in opposition to the beneficial forces of nature called the Gods.
That is the main reason why you seldom find an Asura praying to Lord Vishnu and asking for boons.But there are a few examples of that too.And once the asura does sufficient penance even Lord Vishnu can't deny him a boon.