The Sorcerer's Careful Spell Metamagic states:

When you cast a spell that forces other creatures to make a saving throw, you can protect some of those creatures from the spell’s full force. To do so, you spend 1 sorcery point and choose a number of those creatures up to your Charisma modifier (minimum of one creature). A chosen creature automatically succeeds on its saving throw against the spell.

However there are spells which do not force saving throws such as the bones of the earth spell which states:

If a pillar is created under a creature, that creature must succeed on a Dexterity saving throw or be lifted by the pillar. A creature can choose to fail the save.

An example of when you would want them to succeed on their saving throw but they themself would want to fail the save is this:

You wish to ascend using this spell to get to a higher area but there is an enemy flying directly above you. If they choose to fail on the save they would ascend the pillar with you, this is not ideal. If instead they succeeded on the save, they would end up below you and you could walk around more freely.

How do these features interact with each other; can you really not use the Careful Spell Metamagic on spells with optional saves because they are not forced?

Careful Spell works just fine on Bones of the Earth

When a character is told to make a Saving Throw, said Saving Throw has been forced upon them. In the case of spells like Bones of the Earth, the Saving Throw hasn't been made "Optional": the creature targeted has simply been given the option to automatically fail it. They're still subjected to the Saving Throw, and they still suffer the consequences (which may or may not be beneficial in this context) for having failed it.

So it's perfectly fine for Careful Spell to be applied to this spell, or other spells that have Saving Throws that can be failed deliberately (like Zone of Truth).

There is an intersection of specific rules though

It's not clear whether the Sorcerer using Careful Spell to force a creature to succeed on their Saving Throw cancels out a creature that chooses to fail their Saving Throw, or not. They're both specific rules: automatically succeed vs choose to fail.

At my table I would generally rule the Careful Spell takes precedence, because it's specific to this casting of the spell, whereas the choice to fail applies to all castings of the spell. But I can't necessarily say your DM will rule the same. So while Careful Spell would indeed allow a character to automatically succeed at their Saving Throw, whether it prevents them from choosing to fail is an open question.

Careful Spell can be combined with Bones of the Earth

As you point out, the spell states that

A creature can choose to fail the save.

A creature cannot fail something that it isn't being forced to do. In other words, being able to choose to fail doesn't change the fact that it is still forcing a saving throw.

As such, the Careful Spell Metamagic is a valid option to combine with Bones of the Earth.

So can the creature be forced to save?

Of course, once you combine them, you get the effects of both:

A creature can choose to fail the save.

and

A chosen creature automatically succeeds on its saving throw against the spell.

So what happens? Can the creature still choose to fail?

It is up to the DM.

Unfortunately, both Careful Spell and Bones of the Earth are applying specific rules, overriding the general rules of the game. Determining which one wins is something that DMs are responsible for determining.