Well, if we look up the word "theft," we'll notice it means 'dishonestly taking something that belongs to someone else and keeping it (or stealing, i.e. taking something without the permission or knowledge of the owner and keeping it).'
The key words here are without the permission or knowledge , which contradicts the process of taxation. I mean, you know that they are taking the money from you, but on the other hand, they never ask you for permission or whether or not you are willing to do so
So, it's a theft only by half and only providing you're not willing to do so. If you are, then it isn't a theft at all, IMHO.