Not credited in paper written by boss
As a contractor you may have explicitly or implicitly given over all of your intellectual product to the company. If that is the case then what they did is (minimally) acceptable. But in human terms it may not be proper. Certainly not "neighborly". 

Since you indicate that there is a difference in how contractors and non-contractors were treated there may be some underlying justification, valid or not, for what was done. 

I suspect that the practice is pretty common, but not universal. 

However, it would be reasonable to ask for an acknowledgement at least, and maybe authorship. You can ask, at least. You might be refused and you might be given reasons why refusal is necessary, but you gain nothing from silence. At a minimum you will learn whether it is in your best interest to continue under such a relationship.