Why shouldn't we? It seems an incredibly difficult problem. They have reviewers who make subjective calls on subjective rules. The leeway not only gives the opportunity for the user to improve but also gives the reviewers leeway to flag borderline posts without harshly punishing users.

17 is a weird number but having a number is perfectly reasonable to me.

It's also possible that users could misuse the reporting system, in order to get other users' accounts suspended.

Requiring N distinct reports of a suspension reason would seem to reduce misuses of the reporting system.

The 17-reports threshold might have been found to balance type-1 and type-2 errors, as account removals are costly actions when made in error or as a result of reporting-system misuse.