I assume you mean the point limit that is on the flagging feature? It's there for the same reason it's there on voting: to prevent malicious abuse of the systems. Specifically, the use of multiple accounts to cause trouble on the site, which is harder to achieve when you need to go through approval and gain points first.
If posts could be flagged as soon as the account was made, you could just create five accounts and start removing whatever content you wanted. All you would need are five email addresses, which are not hard to make. The same could occur for voting, except you'd just repeatedly vote yourself.
You also bring up the issue that posts could be flagged incorrectly, which is another factor to consider. It would be a nuisance and devalue the whole process if the flagging system were misused by new users, who may not know when posts need attention or break a rule. It needs some semblance of accuracy to be functional.
And realistically, there isn't a particular need for more people to flag posts. 30 points are pretty easy to get, so there'll always be enough people above the limit to keep watch. Allowing new users into the system wouldn't improve much, and it would create a way for people to damage our content.
Pokemaster might have to correct me on this one, but I don't think there exists a way to flag wall posts in the software we use. It would be a nice feature for sure, but for now, a lot of its effects could be replicated by leaving a message for a mod or editor on their wall.