Is my drill weekend a Excused absence from work?

No you do not have to use paid time off for drills, I would get with USSERA they can help with this.

If your civilian employer offers 15 to 30 days of paid military leave per year, separate from paid vacation, you can and should take the paid mil leave.
If you only have paid vacation and and sick days, you should be allowed unpaid leave for drill.
Your unit should provide you with a generic employer letter that includes your drill and AT schedule for the year, that cites USERRA, specifically: "absence from work for this period of military service is protected by the Uniformed Services Employment and Reemployment Rights Act (USERRA), 38 United States Code, Sections 4301-33."

Your job does not have to pay you but they cannot force you to use PTO. I had a similar issue come up because I work for local government in my civilian job and they are very strict with their time off procedures. Call USERRA [login to see] . They did a great job of explaining it to me and they can help you with explaining it to your HR.