Can OSS tires have new tread from a grinder?

I work for a company that is currently using a grinder to make tread in trailers that have flat spots and no tread. The mechanic says it's legal because they are recaps. Every driver I've asked has said that's insane. But I was wondering if there's actually a violation in the fmcsa book or anything pertaining to altering tires or modifying them. I've refused to touch the trailers. The mechanic said he got the go ahead from the owner and the fleet manager. Maybe they know something I don't? On the surface it sounds outrageous.

 

But what does the "most" part mean? Which ones aren't? Is re grooving a tire at a shop with a tire grover the same as a new mechanic using a hand grinder? My confusion is if a tire has a flat spot and is less than 2/32 from that flat spot, grinding another groove just makes it deeper into the rubber. I wish there was something we could point to that says yes or no. Is this one of those things where one trooper is writing me a ticket but a different trooper is good with it?

 

It was a recap they only use recaps. Ya it was out of service they grinded away the bald spot. All their trailers are getting the bounce marks on the inside because their auto air system is set to 87 I've told them for years to adjust but they won't. So now they started grinded. Here's a picture of another one they haven't grinded that they will. Give you an idea of how bald some are

 

I found na explanation in the fmcsa finally. So regroving is legal like you all said but there are standards. New grove depths, minimum and maximum grove widths. Among other things. Another site stressed the importance of doing it at a shop by a tire mechanic on their machine. Judging by the 25 year old new mechanic out there with a harbor freight grinder I'm assuming I'm right that the new groves aren't with in the standard lol