Was it because of legal reasons?
Precisely. There are a lot of nitwit elders out there, who would either say something slanderous, or who would implicate the WTS as having influence on their decision. A recording would provide damning evidence in a lawsuit.
The WTS presents itself as a bold fearless champion of proclaiming "Jehovah's truth", but in reality, nearly every policy they have is predicated on their utter terror of being sued. The threat of losing money and/or prestige is literally the most terrifying thing they can conceive of.