[jjk166]

According to the oxford dictionary, tax evasion is "the illegal nonpayment or underpayment of tax." Dictionaries don't decide what words ought to mean, they list what people who use the words mean by them. Yes, taken on its own evasion does not require illegal activity (though it does have a much more nefarious connotation than synonyms like avoidance), but when you put the word tax in front of it, that changes the meaning. When the average person talks about tax evasion, they are talking about the crime, and when someone says something is not tax evasion, it is commonly understood to mean it is not an illegal nonpayment or underpayment of taxes. Similarly the word exploitation can mean a lot of things, many of which are not illegal, but when you put the word sexual in front of it then suddenly it refers to a definitely illegal thing.