Bias is an adjective though.
But seriously, this is pretty normal, "biased" ends with an /st/ consonant cluster and those things are getting dropped in speech all the fucking time, I'm willing to bet you do it too.
Now combine this with the fact that nouns can act as adjectives in English and it's pretty understandable why people use this non-standard spelling, and really, it's nothing to get worked up about. It's not like it can cause any confusion.
It's almost certainly going to get more popular too.
I don't want to live in this future, time to vote for Trump