I have recently commenced a position of CAD Technician after being unemployed for 12 months.
I am a fully qualified Structural Senior CAD Technician after 4yrs obtaining the relevant qualifications.
Now I find myself working for a fraction of the salary. Is this legal?

Legal has nothing to do with it. You have an entry level job, so started on the lowest rung of the ladder. You took the job knowing what the salary would be. So, work your way up to the salary you are expecting. If not where you are, after more experience, somewhere else.

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My question is why you would think it wasn't legal. Are you making more than minimum wage? Minimum wage laws are the only legal salary requirements. There is nowhere in this country (or most of the world) that says if you have these qualifications you must get this salary. The US is a free market economy (for the most part). This means that salaries are based on the market, not qualifications. You should know, since you were unemployed for a year, how tough the job market is. That's why you accepted a job at a lower salary.