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?