I would like to hear what people think about this restriction.
Do you always cast a vote when reviewing these things?
It's deliberate, mostly because these are new users, so voting is important feedback to them. There's some offical reasoning on Meta Stack Overflow:
So if you can't vote, you're left with editing as your primary tool in this queue. While I'd love think this was enough, in practice folks vote a lot more than they edit - even on Stack Overflow, there are only 2 people consistently averaging more than 30 edits a day. Editing takes more time and effort than any other action on that list, hence the lack of external rate-limiting. If you're diligently editing all or most of the posts you review, more power to you - and if you're done for the day in the First Posts queue, you might find your skills put to good use by either the Low Quality or Suggested Edits queues.
Voting is the easiest, fastest, and IMHO most useful thing you can do in the First Posts queue - if you do nothing else, please vote. And if you burn through all of your votes reviewing first posts, consider that the sign of a job well done. Thank you!
I mostly agree with the restriction; it's a shame you can't continue to go and edit the rest of those posts, but really it's better if they're able to get feedback where necessary, and there's always (on UX) enough people to burn through the queue, so the posts probably won't be waiting for review for too long just because you ran out of votes.