That's what I would presume is the case. I don't know about the details of licensing regarding Elvis songs, but I can only assume it would have either cost too much or been too difficult for them to license them.
Honestly, though, I just wish there were more songs in general, they don't need to be Elvis songs, it just kind of seems like a missed oportunity to not have them in a game featuring a faction modeled after him (The fact that the Kings don't actually know who "The King" they're impersonating is and just do it because they happened to take up residence in that building is beside the point.) and where several quests reference his songs. Really, they should have included more of the songs referenced in quests, but regardless, mostly, I just wish therwe were more variety in music so I wouldn't need to listen to Big Iron every five minutes. I have nothing against that song, mind you, or even any of the songs in the game, although I think I liked some of the songs chosen in Fallout 3 better, but regardless, the main problem is that even my favorite songs get annoying if I listen to them over, and over, and over, and over, and... well you get the point, and due to the lack of songs that the radio can play in New Vegas, that sort of thing tends to happen, at least in Fallout 3, I could switch over to Enclave propaganda and patriotic marching band songs or classical violin pieces if I got bored with listening to GNR, but in New Vegas, is I switch over to another station, it might well be playing the same songs I'm trying to get away from. Sometimes I just turn the radio off because I don't want to listen to the same song again.