I think arranging transportation for guests isn't required by etiquette. But, it is an extra, nice touch the hosts can add if they want. And, it may be the difference between that guest attending or declining, so if it's quite important to the hosts that this guest be there, I would think they'd want to go that extra step. Kind of like, you should check with your top must-be-there guests before booking a wedding date or general location, in case they have a conflict.
As an example, I don't myself drive. I try not to make a big deal of it and tend to get along on my usual chores fine, but if there's something out of the ordinary, like a friend's party, transportation to this new place and back is something I have to work on. Since I tend not to be very social anyway, it's much more likely I'll just decline to attend, rather than try to figure out the bus or a cab. No hard feelings on my part, it's just that at some point the negatives of attending start to outweigh the positives for me.
A couple of friends, however, tend to just matter-of-factly arrange transportation for me along with the invitation--like, "Your parents are invited, too," (so they'll drive me) or "Jane can swing by and pick you up on her way here, if you want." Although that could certainly become dictatorial, in my case it tends to work out well and makes it much, much more likely I'll attend their event. I definitely see it as a nice extra, however, rather than something the hostess must do for me.