Not at all. It's not forcing anything. It's just selling the seat without first being able to verify inventory. If the operating carrier has space, it will confirm. If there's no space, it will come back unconfirmed. There's absolutely nothing shady about it.

Now an airline (read: UA) may forbid by way of system or policy their agents from doing this because they're deliberately programming the front end NOT to show things that are genuinely available, and they don't want them to override their blocking controls. But it's certainly not illegal.