The free kick and discipline answer different questions
Law 12 distinguishes careless, reckless and excessive-force challenges. A careless challenge needs no card merely for being careless; reckless and excessive-force categories have stronger disciplinary consequences. Other circumstances can separately affect the sanction.
Imagine two late challenges at the same place, both producing free kicks. Their locations and restart direction match, but the manner of the challenges differs. It does not follow that the cards must match too.
Watch the action rather than the protest
A player staying down, teammates surrounding the referee or a slow-motion replay becoming dramatic does not itself select the law’s category. The official assesses the challenge and any separate tactical or other circumstances.
This explains why a whistle is not automatically followed by a card. It also prevents the opposite mistake of assuming every no-card foul was considered legal until the referee changed their mind. A foul decision and a disciplinary decision are related but distinct parts of the same incident. The original comparison isolates that distinction and does not rank a particular real collision or provide health advice.
Questions answered
Must every foul receive a yellow card?
No.
Does no card mean the challenge was legal?
No. A foul can still require the restart without a card for carelessness.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.