The flashiders were tack welded to comply with US law, back in the days when the rifles were being made for sale in the US. That is why the receivers often have US importer's marks on them. If you can get the flashider off, and if it is still in usable condition, there is no reaso nit could not be reinstalled. Sometimes drilling is not necessary. You might get lucky.
what was the reason why the US wanted them welded? What is the advantage or security it brought?
In order to be able to import it to the US, it had to be permanently attached.
and the bayonet lug removed.
The Chinese finally came up with a fix that the U.S. couldn'd figure out. One of the problems of the M-14 design was that stupid flash hider kept shooting loose, so, weld it!
Scott