I wasn't actually sure if Sterling was stepped on - hard to tell even looking at the replay. That said, I thought it was a clear foul and that the defender at least tripped Sterling with his leg.
Sterling exaggerated the contact and the ref didn't have a clear view of the play - this lead to a yellow card against Sterling. Extremely harsh and a total miscall.
Sometimes exaggeration works out - forces the ref to make a call - sometimes not.
Should have been a penalty but if players want to cheat or exaggerate, they can't complain too much when it backfires. Shit decision though.