>> why is this legal  <<

Um ...
A better question might be, "why do you THINK it is legal?"
Have you missed all the warnings here, and on the IRS site, about this ILLEGAL scam, and its probable origin in India or Pakistan?

Do you really think mere laws can stop criminals?

Received the same call tonight around 6:30 PM.  They went on to say a bench warrant was issued for my arrest in the next 30 minutes if I did not pay the amount which I OWED TO THE IRS!!!!  They said they were unable to provide any documented proof because I was under "federal investigation"  because I had never responded to their 3 letters AND an IRS agent came to my home "a few days ago" and I did not answer the door!!!!!!!!!!!!!!!  Also, I was not to "tell any third party about this call".   If I needed to ask someone for assistance to pay the amount due, I was to tell them I "needed the money because I was remodeling my house OR I was going on a vacation, etc."   So I said, the IRS wants me to lie, they had no comment!!!!   When I called  the local and state police, I was told I could not file any complaint because I did not provide them with cash or give them a credit card number.   Please share with family and friends but especially with the elderly and anyone you know with mild cognitive impairment.

Next time they call pretend like you will pay on the condition that you will send a certified check payable to the business only which is supposedly the IRS, not a personal name, ask for the address and say you don't want to be arrested.
Then report all the harassment to the Attorney General's Office and the Consumer Financial Protection Bureau.

If they refuse to give you any information, tell them you will call them back after you call the IRS at it's real number to verify the claim.

There is a local IRS office in the 313 area code, google it.  The number calling you is just some scam artist.

It's not.  The callers are basically extortionists living in India and Pakistan.  Invest in a call blocker.

Said it was my final notice and the IRS was filing law suit against me.....Now I'm scared...... S C A M ! ! ! ! ! ! !