@Magpie_in_aus I have no idea really, but is what crossed my mind when I thought about it.
I'm not really sure whether Kiwis (NZ born, NZ passport), who play for other countries, count as overseas players or not. As I understand it, every Super Rugby franchise (in NZ) is allowed to have two overseas players.
Sam Lousi is without doubt an overseas player. Not sure whether there are others. As far as I know, Uhila hasn't played for Tonga. Matu'u has left. To me it seems, that the Canes have only 1 overseas player other than Lamborn (if he is considered to be one). So legally, Lamborn playing for the USA doesn't seem to rule him out.
I'm not sure why being an international player for another country would make him less attractive as a player for other than (possibly) legal reasons. If it was all about a pathway to the All Blacks, they wouldn't have signed Lousi either. Other franchises wouldn't sign Irishmen, Englishmen, Australians or South Africans either. They'd never sign an overseas player. I think that argument would only be relevant if there was a big shortage of loosies in the ABs (which there isn't) and a good supply of up and comers in Mitre 10 Cup squads that needs to be developed at SR level to address that shortage.
There's also the distribution of different positions in the squad to consider. Including the WTG, the Canes named 9 loose forwards in October 2015. This year they named 8. The number of players in other positions has changed quite a bit, too, by the way. Iopu Iopu-Aso and Vito are no longer in the squad. Toa Halafihi and Reed Prinsep are new in the squad. Halafihi (22, never played SR) could be seen as taking Lamborn's spot, or Lamborn has not been replaced.
Lamborn has said himself in an interview, just after he was named in the USA squad in May, that he hoped to play for the Canes and Magpies for years to come. It seems he didn't know at that time that he wasn't going to get a contract. Interestingly, USA coach Mitchell contacted Plumtree to find out more about Lamborn, before including him in the USA squad. My question is, what did Plumtree think of Lamborn's chances of a new contract, and did he or didn't he tell Lamborn?
This year's law trial's were particularly bad for openside's such as Lamborn (he wasn't used at 6 like last year). He wasn't too bad this year, but far less visible than in 2015. He couldn't play his own game under these trialled laws and apparently couldn't make a case for a SR team to sign him. Maybe the Canes signed Halafihi in the hope he's better than Lamborn. There's no way of knowing.
I hope Lamborn stays around. There are always injuries during the season, and if the Canes' or other franchises' loosie stocks go down, he has a good chance of being called up. But yes, a PRO rugby team in the USA may just pick him up instead. Considering the fact that the American competition is, at this moment, reasonably short, that will not have to stop him from playing for the Magpies fortunately.