The way it's presented in the Core Rulebook pretty strongly implies to me that it is meant to be used that way by players. But I agree, regardless of what the designers intended, a better way to do it is for the GM to make a ruling based on the players' descriptive actions.

The problem for me is that actually, even if you do that, the downtime rules aren't very useful. Downtime activities have specific requirements for time spent to count as that activity, usually measured in days. But I've never had a situation where I have multiple days of downtime. My players and I don't say "I want to spend 2 days making money by sneaking into places and robbing them". More normal would be for the rogue to describe themselves sneaking into one specific house and asking what they find. The way they describe what actions they want to do is on the order of something that lasts a couple of minutes at a time. It's just far more fine-grained than the Activities listed in the Downtime section of the CRB is designed for.