Oh you can wage war on the other side of your marches no problem. You just need a CB if you don't want stab, war exhaustion and AE hits. One possible CB are claims. Notice how you can only fabricate claims on provinces that border yours or share a seazone with them? That's because claims also need a reason. In-game that reason is abstracted because the specifics don't matter, but the usual argument is that you have historical ties to the region and those people deserve to be ruled by their legitimate ruler (IE: you). It's kinda ridiculous to claim that a province in the middle of a country should be yours, but some border province? Sure, that even happens regularly through the Border Dispute event. That's why claims can only be fabricated on your borders.
Now on to our disagreement. You claim that you should have the right to fabricate claims on the borders of your vassals. Poland barely has any legitimate reason to govern the people of ryazan, but Lithuania with their acceptance of various russian cultures could very well argue in support of such claims. So why would it be possible for a polish claim to exist on the other side of Lithuania in the heart of Russia? Denmark/Sweden provides another example.
Now these are PU's you might argue. A legitimate argument, maybe there are some subject types for whom this doesn't apply. But the trouble is that subject type has no relevance to the size of the vassal. I simply used PLC and Scandinavia as examples because they exist in 1444 and are well known. I've had many a Russia game where I used Qasim to eat the Golden Horde + Nogai while I toyed around with PLC and Sweden. When Qasim is bigger than the Ottomans, it makes no sense for me to be able to make claims to land across that country. There are infinitely many scenarios imaginable where it makes no sense to claim land across the vassal for yourself. That vassal claiming land they themselves border is perfectly normal, but the parent country itself has no legitimate claim to that land.
