There are a number of charges that you can be arrested for at the sole discretion of the officer. I've heard exchanges like:
officer: Don't go anywhere
friend: Am I under arrest?
officer: No, but don't go anywhere.
friend: If I'm not under arrest, then I'm leaving.
officer: If you leave, I *will* arrest you with impeding an investigation / obstruction of justice
(That exchange followed from someone not involved in a protest, who was filming police officers trying to contain a protest. They tried to fine her friend $400 for littering (i.e. a candy wrapper that was already on the ground) or something equally ridiculous when you consider the amount of time (and tax-payer money) that they wasted harassing her and her friends.)
Also, in a number of jurisdictions, you can be arrested for being 'drunk and disorderly' with only the officer's word that you were actually drunk and/or causing a scene.
Keep in mind, that it doesn't matter much whether or not the charges stick. The police have successfully harassed you if you have to spend the night (or weekend) in prison before you are able to get out (and/or get the charges dismissed).
{update} Some corrections. Also, the 'investigation' that was being impeded was a $400 fine for littering a candy wrapper (ignoring whether or not the candy wrapper was actually littered).
Your first example is naive. The police cannot arrest you absent an actual crime, but they can detain you for a "reasonable" amount of time; if you simply walk away from an officer, you are in fact impeding an investigation. There is a clear difference between an arrest and being "detained": during an arrest, the police can search you and your vehicle, put you in restraints, and drive you away to a police station.
That there are subjective offenses you can be arrested for and that the police could abuse strikes me as a simple fact of life. You can also file complaints, (in most jurisdictions) record the abuse with your camera phone, and (in crazy cases) sue.
In my example, the people were 'detained' for a few hours, during which they were not asked any questions at all. They just had to sit around at the police station twiddling their thumbs. In my mind, this just police intimidation. Some police officer didn't like them or what they were doing, so they decided to harass them. I don't think that, as a society, we should just accept this as a fact of life. By doing so, we give the police carte blanche to continue doing so, and to attempt to push the envelope even further.
If a cop ever pulls that on you, lawyer up instantly. If you're not free to leave, you're under arrest, regardless of what state they say you are in, whether they have mirandized you, etc.
Again, you can be "not free to leave" for a whole hour on the side of the street and have a judge call that a "reasonable" period to detain you for questioning. You're not under arrest while that's occurring. A bunch of other processes get set in motion once you're arrested, all of them bad.
Probably don't overreact if you hear a cop say "stop, get back over here!" or "no, you may not leave until I get to the bottom of this!"; they're legally authorized to do that pretty much on a whim.
If you've been handcuffed or transported or confined in a police station, you're under arrest, and yes, demand a lawyer.
Can you say a bit more about "lawyering up"? I'm not sure what you mean. Do you mean specifically to call a lawyer, or imply that you have one... etc. Thanks.
Also, in a number of jurisdictions, you can be arrested for being 'drunk and disorderly' with only the officer's word that you were actually drunk and/or causing a scene.
Keep in mind, that it doesn't matter much whether or not the charges stick. The police have successfully harassed you if you have to spend the night (or weekend) in prison before you are able to get out (and/or get the charges dismissed).
{update} Some corrections. Also, the 'investigation' that was being impeded was a $400 fine for littering a candy wrapper (ignoring whether or not the candy wrapper was actually littered).