People are confusing 'security clearance' with 'background investigation'. Non-citizens very rarely have any US security clearance. It is normal for Army units

People are confusing 'security clearance' with 'background investigation'. Non-citizens very rarely have any US security clearance. It is normal for Army units to have many non-SECRET cleared members. It is one of the challenges of distributing ISR information to frontline units. Signals (communications) will tend to have clearances, and they couldn't progress if they couldn't get one. Officers are cleared.