Either way i just think that through psn network they should allow for all consoles that COULD contain the game on that system.. Like for ps3,ps4 or psvita..instead of having to buy them individually on each system.. dl service or no dl service.
You CAN buy Vita games through the PS3 and PS4.. But not vice versa. And you can't buy PS4 games through the PS3. Vita games can be bought since they can be stored on your system and then transferred over to the Vita. The Vita does not have access to the PS3 and PS4 games since the system itself can't play them, and definitely can't hope to hold most of those games efficiently (especially not PS4 games). Same goes for why PS3 can't buy/download PS4 games, and hell, one PS4 game is bigger than what the PS3's Blu-Ray disc can even hold (Blu-Ray disc is 50GB, Killzone: Shadow Fall for PS4 is approximately a whopping 60GB).
That's... my theory, anyway. But it makes a lot of sense when put like that. I'm pretty sure you can buy for every platform through the PSN website, but I've never done that so I don't know.
Besides, why would you be buying PS4 games without a PS4? And if you have one... why would you be buying them with one of the other systems, and not through your PS4? etc. etc. On top of all that, if every game for every system was available on every version of the PSN, the PSN itself would later on be pretty damn cluttered.
Also, while I
would like the ability to play PS2 games on my Vita, that would require them to do more patches and stuff to make the games compatible with the Vita (due to the Vita lacking L2, L3, R2, R3, pressure-sensitive buttons, rumble might be an issue, or even the game itself might be an issue due to being a PS2 game).
tl;dr... The PSN works the way it is. Just a bit of a hassle to navigate on PS3.