So I get the basic rules for grappling, as far as I can understand any abstraction that requires a large flow chart. Here's what I don't get-
Why can someone who initiates a grapple use a full attack? Meanwhile, the defender, who is being grappled, gets to keep the grappled condition while still making a full attack? Why must I spend a standard action (or dump feats to make it a move action and get two attacks) when I am the one who is holding onto the other person or creature in question? It seems... stupid.
So here's the deal- I'm playing an unarmed fighter, and I want to grab things and punch them. Seems straightforward. I want to hold them while punching. But the rules seem to disagree, and tell me that somehow when I choose to grab onto an enemy, I am really just screwing myself over because the enemy has more damaging options. If I ever fight myself and play realistically and grapple first, I screw myself, allowing the shadow-me to punch away as I would like to do in the first place.
Am I really penalized for having initiative?
Yup! Really it doesn't work out until you start into the greater grapple and rapid grapple feats, and your opponent is much worse off when they are pinned.
dont' forget that they can't use 2-handed weapons while grappled, which can drastically reduce their damage options. i'm not sure what is RAW/RAI, but 2WF could very well be barred as well. if they are a caster you are also forcing them to make a seriously hard concentration check to pull a spell off - even if it's quickened/stilled/etc.
grappling non-humanoids with lots of natural attacks isn't a strong option (unless you can pin them quickly, e.g. greater grapple) since as you mentioned, they can full attack back with full effectiveness... but that is just one case, and between the other cases i mentioned, there's plenty of situations where it can be useful.
Am I really penalized for having initiative?
In so far as grappling, you are being 'penalized' for trying to take a foes options away. You want to stop someone from taking meaningful actions, take out their HP.
Being grappled hurts mobile targets and two handed weapon wielders, but a one handed weapon wielder has it's fair chance of gutting you. Think of their full attack following up your grapple as them driving their sword into you nice and deep as you pulled in after you got the initial hold. Sure they didn't stop you from getting the hold, but hoo-boy did they run you through as you were clinging on.
The way d20 runs surprise rounds and full attacks DOES penalize you for winning initiative, but that is another topic.
maguses should also not be able to do their cast+attack thing since that requires 2 hands to do.