Your definition of theft is not accurate.
At Common Law, theft is defined as, "the act of converting chattels to one's own use without colour of right."
Conversion does not require taking. A bailee commits the actua reus of theft when he refuses to return the chattels of the bailor, thus coverting them to his own use.
At Common Law, theft was limited to chattels, so when intangible property came to be understood in law, legislatures generally stepped in and codified the definition of theft in statutes. These vary from jurisdiction to jurisdiction, of course, but no force coercion or fraud is necessary for a person to commit "theft of a telecommunications service."
Equally, there are plenty of circumstances in which property is taken through coercion or force which are not theft, because colour of right exists. When I get a court order to compel you to return property that belongs to me, and I send the bailiffs to recovery that property--it is not theft. I have colour of right, because of my court order, and the bailiffs are acting lawfully for the same reason. When you pledge a security interest in your property in order to borrow money, the lender may, if you fail to repay, use force of law to recover that property from you. That is not theft, because your act of pledging a security interest created a colour of right upon your default.
Even if you don't want to rely on a Common Law or statutory definition, a plain language definition will still include the essential requirement that the taking or conversion of property must be felonious or unlawful. This is an essential characteristic of theft: the taking must be unlawful.
It follows that anything act that is authorized by law cannot, by definition, be theft. And tax (and its collection) is authorized by law.
quod erat demonstrandum
I also take some issue with your definition of tax. A tax cannot be levied (in democratic jurisdictions) without legislative approval. That battle was waged and won during the English Civil War, and it has been established ever since that the only legitimate origin of a tax is Parliament. Your definition speaks only to execution--the methods upon which government may rely in order to collect tax--but it does not define the tax itself. I would be inclined to define tax as, "the levying of money obligations upon persons or property by lawful authority."
You are tying to prove your assertion by creating parallel definitions. This is logically sloppy, and it is rhetorically dishonest.
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--James