Third seat definitely can take liberties (both weak and strong). You don't have to worry about missing game, and you can preempt on hands that you never would if partner could need to judge how the hands fit for game. You can open with "a reason", and expect not to be hanged for it.
However, those liberties have to be the focus of agreements, and can include changes to system to handle them. When it goes p-p-1S-x; 2S-3H-p-4H, partner still has the key decision to make. If they expect you to be dead straight, they will double (for -790), or bid 4S (-800 into -620, or -500 into +100) if you weren't dead straight. If they expect you to take liberties, they will pass 4H on hands that really should either double to set or sacrifice, because this time you do have the hand. Worst of all is when both partners insist they are right and refuse to come to an agreement both players will stick to. Note that that agreement could be "M takes liberties a fair bit; E is pretty straight."
There are even conventions such as Reverse Drury that ask the question: "Do you actually have a hand this time, or are you playing games again?"
As far as what liberties to take, I personally believe you should have a good reason for whatever you do. So if I open 1M with a nonstandard hand (usually a 4-card suit and subminimum values), it's either because I want to preempt the opponents, or I want partner to really lead that suit (KQTx or something like that). I hope partner can figure out which one it is when it comes time for the opening lead. If I preempt with absolute nothing, I've worked out the numbers already. If I preempt with an 'opening hand', it's not the kind of hand that will hate to miss game opposite some random 11 (and would be willing to try for game opposite 4-card support and little else).
Fourth seat bidding is very interesting, because if you open, you intend to go plus. If you open with a 2 bid or a 3 bid, you still intend to go plus; so it's not a 'preempt' in the standard sense of the word. I hate passing out hands, because I never know how they're going to score. At least coming in and getting a minus means I have an idea (even if it's "likely bad"). I do it when it seems right, of course. But again, partner knows what you have when you bid.
Specifically, Pearson points (the "rule of 15" you mention) for fourth seat isn't preemptive at all - it says "the more spades you have, the more likely you will go plus, because the side with the spade fit can compete over the opponents without going up a level. The fewer spades you have, the more strength you need, because they are more likely to have the spades."
But in answer to your title; as you can see, not necessarily. A pass (playing a standard system) still contains 30% of hands, nowhere near limited enough that you're in sole control. You just have a different/wider system available to you.