I've looked up in my Oxford Advanced Learner's Dictionary and New Oxford American Dictionary, it turns out that the difference (if any) between "protest sth" and "protest against sth" looks very hard to tell. Examples are as follows:
- Students took to the streets to protest against the decision.
- They fully intend to protest the decision.
- Doctors and patients protested against plans to cut services at the hospital.
- The workers were protesting economic measures enacted a week earlier.
In addition, I'm wondering if the object following them really matters to the choice, but examples 1 & 2 seem to prove me wrong. Any more explanation on this?