Well, they can learn our defensive schemes and get into NBA shape whether they are signed or not. If they are not doing that, then they are making a very bad decision.
I am guessing you are referring to the fact that Giddens agent told him not to participate in the FA camp. That was different than going to team workouts, or working out in general. He didn't want him to go there unsigned, and get injured...not to mention, he wanted to tweak the C's management a bit.
Unsigned players ofter work out with the team prior to signing the contract. Right now the C's are just trying to work out all of their options. There is absolutely no rush to sign these guys, since they have them in their control no matter what, but as soon as they sign them, they lose some of their flexibility (they have to wait 30 days if they want to trade them).