The free-agency market, like any other, operates under the law of demand and offer. If the offer is scarce, prices will raise. This free-agency pool was extremely weak. That's why every serviceable player got a big contract: Diop, Beno and Posey got the full-MLE; even Duhon got a $11M/2 years contract.
First, why would you spend money for the sake of spending money? Second, just because the Knicks made a colossal mistake in overpaying for Duhon and the Beno/Diop received ridiculous money, does not mean the C's/Hornets should. Don't let other teams dictate the market. That is simple management. It gets teams into trouble. Also, you leave out that Duhon's deal was only for two years. The C's (presumably) offered Posey the full MLE for three years, which is more than reasonable considering his age.
The FA market isn't as thin as you claim. The C's are an attractive FA destination for buyouts during the year and giving Posey a 4th year, when he will most likely not be productive for the third/fourth year just makes for a solid business decision.