The following is a excerpt from the Unwritten Rules of Baseball*
Rule 69 (a).
When an opposing pitcher is down 3-0 in a count, the batter will take the next pitch, allowing the pitcher an opportunity to work their way back into the count
This has been a common practice in the game of baseball since forever. Pitchers will fall behind in a count, 3-0, then the hitter will give them the chance to just get a ball over the plate for a strike. After all, the hitter still has every chance to wallop a 3-1 pitch and if the pitcher happens to miss with their 3-0 offering, they get a free pass.