Jackson was a free agent. He was signed in free agency. It's a new contract, not an extension. Davis, Lillard, and Drummon are not free agents and thus only eligible for extensions, which is why they should not be compared with Jackson.
It's semantics, but I hate to be wrong. Jackson was a restricted free agent, but he signed an "extension" with his own team. That is, of course, different than Drummond, who doesn't have that option yet.
http://www.freep.com/story/sports/nba/pistons/2015/07/05/detroit-pistons-reggie-jackson/29752173/
https://twitter.com/WojYahooNBA/status/617873845161037824
A couple of things:
1) You're losing the point of the semantics by not quoting upwards. The suggestion was that Drummond would be available because he might somehow be angry that Jackson got a new contract before him. The point being that Jackson was a free agent, so it's not surprising he'd get taken care of first.
2) it was not an extension, by NBA lingo. Wok is imprecise with CBA terms, especially on Twitter. And the Freep.. Well, they're imprecise in a lot of their sports coverage, and I'll leave it at that. But an extension, in NBA language, and really contractual language, is an addition to a previous contract. Jackson is no longer under contract with Detroit, and is free to negotiate with any team. Any new deal is not an extension. Also, this is important in the CBA, as there are only specific times extensions can be signed. For a rookie deal, it is after the third year of the rookie contract is up, from July 9th (tomorrow) until October 31st. Jackson did not sign such an extension last year with OKC when he was eligible, because they could not come to an agreement. So he entered restricted free agency, and remained with the Pistons, who used his Bird rights to give him a fifth year in his contract. Drummond is eligible to sign an extension between now and Halloween. Because of the massive projected cap spike next year, I'd be surprised if he came to an agreement with the Pistons before next summer.