Whether something is an exploit or not is entirely up to the player. Every person has his or her own cheese-meter. On mine, this tactic has some very mild cheese, but isn't quite as cheesy as other ways to beat the AI at the diplomatic game. You're giving the AI something it actually needs in return for something it can't possibly defend anyway, a fair deal in my book and there are way way worse ways to convince the AI to give you planets and an unfair advantage. (Although, if the ships are so bad that they are in no way up to the enemy they're facing, the cheese value goes up, of course.)

Buying the last planet off of a civ is something I do frequently, I defend this course of action by telling myself they have "joined" my empire in order to save themselves (which gives me access to their tech and ships) in return fr a sizeable gesture of good will (the ships/tech I sacrifice).

If you're in a position to "exploit" this tactic, you're probably already the strongest race in the galaxy and by default the winner (afterall, you could go on a conquering spree with those ships you're selling/keeping yourself safe with) unless something rather strange happens. This tactic becomes a lot harder to execute on the higher levels (while still being effective at it), which, again in my book, also lowers the cheese-factor. If you're having an easy (and boring) time with this on Tough, I suggest upping the difficulty level as you're having way too easy of a time.

In the end though, what is an exploit and what is not, only really depends on your own opinion on the subject. If you feel something is gamey and ruining your immersion/having a good time, then don't do it! If youcan justify it to yourself and/or within the game lore, eat your heart out!