The idea that 'taxation is theft' is one of those thrilling, paradigm shifting recognitions that are continually being rediscovered and shared. It is a classic case of pseudo-intellectuality, in which the excitement around an idea determines its popularity, rather than its quality (previously).
First, unlike theft, taxation is legal - and this turns out to be a more significant difference than it first seems since we rely on laws to determine who owns what. Second, taxation is a device for solving collective action problems and thus allowing us (by coercing us) to meet our moral obligations to ourselves and each other - including our obligations to respect each others' property rights. One can't coherently be in favour of enforcing property rights, e.g. by having a police force and judges to catch and punish thieves, without also being in favour of a sustainable system for funding that enforcement.