Brake drums can be machined and turned if they have some groves in them.
However most times it costs near as much or more to turn or machine brake drums than it does to just install new new brake drums.
For example the brake drums on my truck would cost $25.00 each to machine but I can buy new brake drums for $40.00 each.
So for me it would not make much sense to pay the money to machine the brake drums when I can pay just a bit more money and have new brake drums.
Brake drums typically last for 100,000 miles or even longer so brake drums don't need replaced all that often.
For larger more expensive brake drums such as for semi trucks, buses etc then it might be worth the cost of machining brake drums.
But for regular vehicles it's usually not worth the savings to machine brake drums.