Why does the transmission fluid pour put after removing drive shaft?

It better. You have a problem when there's no fluid in there. There's an axle seal on both sides that seal against the inner cv joints. If you're just working on one side, jack the car up so that side is higher. That will reduce how much fluid runs out. The inner cv joint is the "plug" that keeps the fluid in. You removed the plug, so yes, in that case it's normal for the fluid to fall out. If that happens with the half shaft installed, then that's not normal; the seal would be leaking.

If it's an automatic, look for the dipstick. You fill it through that tube with a funnel.

If it's a manual, there's usually a plug on the side to unscrew. Typically there's two; a lower one to drain it and the upper one to fill it. That can be a messy job. Mechanics usually use a siphon tube to draw in fluid from the container, then to push it into the transmission until it's high enough to run out.