What do you think about promotions while on inactive reserves after EAS?

Dude, as bad as it sounds, some (very few) Corporals in the active reserve component pick up corporal undeservingly (In My Opinion) . That's just how it is. They pick the most qualified people from the unit (on paper) and make then leaders. Don't sweat about how you attained your rank so much as you do the responsibilities that come with it.

I'm not a 100% but I'm pretty sure when you pick up rank while a apart of the inactive reserves you are not taking a quota away from active duty. Because there are separate boards and promotion authorities for active duty and reserves

If anyone should be upset at all (and I'm not necessarily saying they should be), it would be the Corporals who had the E5 cutting score their last month of service, and were not promoted. Granted, the TIS requirement is 4 years for E5, which essentially means the active duty Corporal put in his 4 years, and the Prior Service Recruiters try to dangle E5 in front of you as a tool to get you to reenlist. If the logic NOT to promote those Corporals exists, then the same likely should have applied to you. I'm not saying you should not have gotten yours, I'm saying we should have gotten ours.

That said, the E5 rank is usually flooded with people with anywhere from 4+ years in up to some maximum number of years where they often end up passed over for E6 twice and then are booted out. That has been a lot of years for some Sergeants. I'm sure somewhere within that mess, they are just limiting the numbers of potential 20-year pensions they'll ever have to pay out.