Is it Legal?
Bottom Line the unit cannot force a Soldier to buy a 2nd set of ID Tags if they were not issued or if the unit did not require the items to be brought. Usually the ID Tags are supplied during RFI, Pre-Deployment Health Assessments and SRC.
Let’s Look at the Issue from both sides
Chain of Command’s Side:
Apparently there is a need to ensure each Soldier has 2 sets of ID Tags. Now these are typically ordered by the unit. The unit turns in the proper paperwork to the nearest military personnel center and they are produced locally. Since you are deployed this could be an issue because the capability to produce the ID Tags may be located a long distance from your position.