Obviously you shouldn't be used as an RA, and if this is what your supervisor is doing then they are in the wrong. I would suggest a couple of things however. Most importantly, speak to your supervisor about it (most problems seem to come down to a lack of communication). They may not realise how time consuming the work they've given you is, they may also think it is relevant to your PhD and/or think that it is a good lead in to the subject that will prepare you for the work you have ahead of you. Of course they may know exactly what they're doing and are getting a cheap RA but I actually doubt they are doing it on purpose.
So have a meeting with your supervisor and ask them:
- How much of my time should this be taking up, as I think I need to spend time on my main PhD topics?
- Is this work relevant to my PhD? If so how?
If it turns out it's not relevant and/or is kept at a level that prevents you doing your PhD work then you're probably best off making a stand now, before it becomes the accepted status quo.
Obviously that's just my view (without any inside knowledge of the problem). More detail would be good, as would the opinions of others on here.