Xenophonz,
I have family who are or have been plain clothes cops and pub bouncers.
And from what I've heard of US law too, MOST places, for a store detective to do that he'd get arrested.
They are only allowed to:

a) Prevent harm, in which case anything goes within limits of the threat offered.

b) Assist in an arrest, they have no right to detain, per se, only a police officer can do that. the security guard perform duties FOR the store, so someone from the store has to press a charge of theft. They can hold you on that accusation until a cop comes to arrest them. Isn't that correct in most states?

In Scots law that's actually a very tricky point, as there's no such thing here as "Citizens arrest". You can "assist a constable in perfoming his lawful duties" though and that and "acting to prevent a breach of the peace" is how security guards holding thieves etc is dealt with in law, IIRC.
Just a note if you're ever over here and get in trouble ;)
Holding someone prisoner =- unlawful imprisonment and gets you jailed, here.

In general, using force like that will get thier ass canned, if not jailed if someone complains.
There is NO excuse for it in the circumstances described, even though, oh I do know, dealing with scum leaves you very dispeptic regarding the two-legged vermin :/

Now, I won't talk about what actually occurs at times regarding junkies and sucmbags of the worst ilk...rough stuff that's not allowed does happen.
It's not kosher, it's ugly but sometimes, it' s needed. Sometimes the only way to stop a scumbag escalating their actions to serious violence, is to take him out back and hospitalize him before he does that to someoen else. That's a reality most folk don't understand, and shouldn't have to.

No idea why people think gangsters, criminals and psychos are "cool/sexy", they are vicious, mostly cowardly, insane, brutal, selfish, venal thugs who'd all be much better pushing up daisies or welded into a cell. They are genuinely evil, mental cases etc, very few of that worst sort are ever redeemable.

But to give a woman a repeated beating like that, uh uh, not the same league at all ! You don't hurt someone like that without one hell of a good reason, theft from a store is NOT one of them.

 definately a case of either the store security guy gets fined/jail time for serious assault, or, the victim gets a big fat pay cheque to drop charges, and the security guy gets fired.

Someone stealing something from a store is not worth jail time, getting a knife in the riibs, being pricked by a junkies needle, or beating someone up over.

Also...lot of folk genuinely, absentmindedly walkout with things without paying. bet all of us have done it once, lol. Not theft, just foregetting you have put something in the wrong bag etc.
Another reaosn why you don't over react.

"I'd rather be a Fool who believes in Dragons, Than a King who believes in Nothing!" www.silverblades-suitcase.com
Free tutorials, Vue & Bryce materials, Bryce Skies, models, D&D items, stories.
Tutorials on Poser imports to Vue/Bryce, Postwork, Vue rendering/lighting, etc etc!