Is the "will" in "can and will" necessary?

Solution 1:

The will in can and will is not the epistemic ("predicted/expected future") will, but rather the deontic ("be willing to") will, the one that shows up in hypothetical clauses (where you were probably told in school that you "can't use the future tense").

• If he will hand in his homework, I will correct it.

The first will is deontic only; the second could be either deontic or epistemic.
(Ambiguity, too! Aren't modals fun?)

That means that, since the can part means be able and the will part means be willing, the whole phrase can and will is just a verbing of able and willing. And both parts are necessary.

Modals are tricky that way; every modal auxiliary has an epistemic and a deontic sense, and you always have to figure out which one is intended, because they're not marked and each reading of each modal has its own unique grammar.

Solution 2:

Strictly the can is unnecessary, as it's entailed in the will; anything one will do one by definition can do, while not everything one can do one will do.

However, considering the context, there is a value in making the two statements severally:

We have the legal right to...

We have a policy of doing...

While one would obviously hope that people would not have a policy of doing things they had no right to do, in a context of stating different people's rights in a given situation, it's worth emphasising.

Solution 3:

As a matter of grammar, can is unnecessary (except for extra emphasis) as it is implicit in will (as @JonHanna says).

On the other hand, it has always struck me (as a Brit.) that the will is strictly incorrect. I imagine that the person being questioned may well say many things that subsequently are not in fact used against him: either because they are irrelevant or neutral, or because the prosecution choose not to use them for whatever reason.

I've always thought it ought to be:

... anything you say can and may be used against you in a court of law.

But legalese is notorious for stating the obvious, repeating itself in different words, using ten words when one will do, and trying to avoid all possible loopholes. They particularly want to avoid a lawyer arguing that something their client said cannot be used in court because the suspect wasn't properly warned.