A much better analogy would be for the postmaster to be noting the sender and recipient of every letter that came through his station in a log that was then passed along to King George's agents in the colonies, allowing them to draw whatever inference they might like — while somehow also having a copy of every letter to hand for further perusal in the event any of the corresponding parties were deemed "suspicious."
I'm no scholar of Revolutionary era America, but I'm pretty sure that if shit of that sort were going down, it would have gotten specific mention in the Declaration.
I'm not sure precisely sure what shit you're talking about going down, but the Crown certainly brought a lot of shit down. Consequently the Constitution certainly respects a Freedom of Assembly. And yet at the same time they doesn't go so far as to forbid the government from tracking and monitoring assembly.
If you associate with someone that runs afoul of the law, why is it unreasonable to consider you? You seem to think the only possible outcome of actually examining your shit is incrimination. It cuts both ways, you know. You could be ruled out.
I'm no scholar of Revolutionary era America, but I'm pretty sure that if shit of that sort were going down, it would have gotten specific mention in the Declaration.