I think the "no assaulting" (unless it's an assault/open-topped vehicle of course) provision is possibly meant to counteract the additional movement that almost all vehicles have now, and also to increase the amount of time that infantry have to spend on the board. This doesn't necessarily mean that GW want to nerf assaults or to buff shooting in a general sense--though it seems they did slightly--but it may be to say that they found it a problem that games would regularly pass in 5th where models ostensibly in the army were either not on the board at all, at any point in the game, or were there (in the case of a lot of assault units) for around 2 turns. At some point, in extremis, this is going to make people start to question why they're spending lots of money and time on infantry models.
Then there is the general principle that, in every edition save 5th, 40K has always been primarily an infantry-focused game and transports, as others have said, have been just that: Ways to get from point A to point B, not ways to protect units from shooting all around (though they are of course meant to protect against "small arms" fire, and still do, unless it's Gauss or pulse rifles, or they're AV10, which is a small step above your average car or truck and, in that case, a high-powered rifle/40K equivalent
should be able to damage them).
Anyway, it's there in the name: Transport. Rhinos for example are still effective ways to get Tac squads and the like into the mid-field where they're going to be able to maximize the impact their most useful weapons (bolters and plasma, this edition) as well as fight over the most highly-contested objectives. IFV-type transports (i.e. those with heavy weapon turrets, more or less) may seem horribly hurt, but the truth is that lot of them were a decent value just for their mobile shooting anyway. This was one of the main problems with Razorbacks being way too good in 5th (and if you don't believe me, the prevalence of Razorspam lists in any tournament, with any codex that could take them, is probably sufficient evidence): If they had been horribly inefficient at shooting but could act as a de-facto fast, scoring unit with AV11, that would have been one thing, and if their cargo had had to disembark to accomplish anything, it would have been fine for them to be good at shooting for their cost, but the combination of both was game-breaking in the sense that it colored the competitive meta very, very heavily. In 6th, you either take the up-gunned version for the guns, or you don't take it. Fortunately, Chimeras and Razorbacks (and to a lesser extent Raiders, Venoms, and Wave Serpents--though Wave Serpents are also paying for superior armor to the extent that they're basically a shooting tank as much as a transport) are still really decent deals for their guns and even if transport capability is just a minor bonus, I think they're still worth it. Improved ability to keep on shooting even when being subjected to enemy shooting is much more a boon to them than it is to a Rhino, obviously.
For example if you compare the cost of a twin-linked lascannon Razorback to a Predator with twin-linked lascannons, you're saving quite a lot of points and the transport capacity is just a cherry on top (I know it also has much worse armor, but still, it is
much cheaper). This is not to say that 6th Ed. Razorbacks are awesome, but they're roughly as awesome as (non-BA) Predators, which is to say they have their place but are nothing to write home about. I think Chimeras fair even better because their weapons became more useful (S6 kinda sucked against vehicles in 5th, and is now solid), and the various Skimmer transports benefit from a "permanent" (unless they're immobilized

) 5+ cover save to compensate for their relative expensiveness, and the fast ones can fire all weapons at full BS while moving short distances and gaining said save.
(Before any Eldar or Tau player jumps on me for this, I do think your transports are still overcosted; just not as much as some people may.)
TL;DR: Non-gunned-up transports, due to being faster, remain effective ways to get squads to the middle of the board faster than they would be able to without transport, and probably with less damage taken; if the Rhino 'splodes immediately after its job is done, it's still done, and if people ignore an empty Rhino it can still bounce around doing irritating things. The transports with numerous/effective heavy weapons should be looked at bargain FA/HS choices that don't take up a slot first and foremost, and as transports second, and with this view they will seem less disappointing.