I think the policy could be placed in a more obvious spot. The text in the signup form would probably not be read by some impatient people
Pretty much so (mea culpa).
Anyway, the simple fact that there are multiple posts on the web frustrated by this issue (just do a Google search for "bitcointalk can't post") is I believe a sign that an usability problem does exist and it is faced by a non-trivial amount of new users.
What's even worse, this problem most likely disproportionately affects organic new users (such as those coming from Google traffic) which simply want to contribute to a thread they have found. Spammers, scammers and sockpuppets are not really affected since most likely new accounts created for this purpose belong to already existing users.