For the same reason they use ancestry databases to narrow the field of potential serial killers to a few that can be investigated through legally-admissible means.
There is a vast gap between what the cops can do in general and what holds up in court, but to a first approximation: the things they aren't allowed to do because it would taint the case are in general explicitly spelled out, and if it isn't spelled out it's legal to use as a stepping stone to conventional, more-understood-protections police tactics. Thus arresting someone for a general Twitter post is probably off-limits (the incitement-to-riot or sedition laws are narrowly tailored), but taking someone online who says "Hey let's all get together and do a riot" seriously, and allocating police resources to prepare for it as if they're telling the truth about their intentions, is almost certainly legal.
(This is the battleground that the ACLU fights on in this day and age).
There is a vast gap between what the cops can do in general and what holds up in court, but to a first approximation: the things they aren't allowed to do because it would taint the case are in general explicitly spelled out, and if it isn't spelled out it's legal to use as a stepping stone to conventional, more-understood-protections police tactics. Thus arresting someone for a general Twitter post is probably off-limits (the incitement-to-riot or sedition laws are narrowly tailored), but taking someone online who says "Hey let's all get together and do a riot" seriously, and allocating police resources to prepare for it as if they're telling the truth about their intentions, is almost certainly legal.
(This is the battleground that the ACLU fights on in this day and age).