Here's the thing, there isn't anything, except for a charter's own rules, stopping someone from diving solo on any given dive. I don't know how things work in the states, because i've never been diving there. But, in the UK (at least with the charters/shuttles I have used), as long as you have some sort of redundant gas source (twinset or pony), it doesn't matter if you have a buddy. In the early days of the sea diving season last year, my buddy had a catastrophic drysuit failure and called the dive at about 4m on the shot. When I got back on the boat after my mate, the skipper asked why I didn't continue the dive as I had my twinset. Truth is, if conditions on the site had been better, I would have done. It was a site I know well (normally dive it a few times a year off of the same boat), shallow (13-15m depending on tide), and a generally easy dive. But, it was pitch black at 4m and vis was pants, so decided to board the boat with my mate.
The thing is, if you have to ask if it's okay to dive solo, it's not something you're ready for.
The thing is, if you have to ask if it's okay to dive solo, it's not something you're ready for.