It's not just for loops. While loops are the same way.
According to the JLS, you can have exactly one 'thing' in the body of your loop. that thing can be (for example) a for-loop. Or an executable statement. Or a block of code surrounded by curly braces.
So, your first loop has one thing in it - a for-loop.
Now...just because you CAN leave those braces off doesn't mean
you SHOULD. Pretty much everyone here is going to tell you to ALWAYS use them, whether they are strictly needed or not.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors