ummmm ... because it is funded by taxpayers solely for corporate benefit? It's especially insulting that it's used to unemploy American taxpayers to satisfy corporations' desire for cheap labor.
That is just a left-wing buzzword used against companies they do not like.
Corporate welfare is government/public assistance to corporations for the sole benefit of corporations. REAL conservatives use the term corporate welfare too -- Tom Tancredo, PAt Buchanan, Bob Barr, Tim Hutchinson, etc.
Corporate welfare is as offensive as individual welfare and should be stopped in a truly conservative environment. REAL conservatives oppose corporate welfare.