Two things:

1) In Oklahoma where I went to high school, they took _in loco parentis_ fairly seriously. This is the legal principle that puts the schools in place of the parents of the child - which means that they are allowed to do just about whatever that the parents won't sue over, including what other state agents can't do - searches, etc. But then OK is strange - two examples: When I was working as for the state (and no, it wasn't in a orange jumpsuit), I had to sign an allegiance form for the state; and when I was in elementary school in the late '70's, I remember a couple of older women were allowed in to do a Christian missionary-type session.

2) By participating with another entity - anything from a concert venue to a retail store - people are entering into a contract relationship with that entity. Unless its a clear violation of consumer protection or civil rights law, they can specify just about whatever they want: No outside food or drinks, All containers are subject to search, No recording devices or flash photography, You are being video taped for your safety etc, etc, etc. Usually there are 'conspicuous' signage but some of the legal language that most people don't notice are the backs of tickets or on receipts.

Personally, I don't really mind most of the actions of the entities that I'm dealing with because I they have the things that I want and the benefits of participation outweighs the inconvenience. If the return doesn't outweigh cost, then I won't participate.

Oh and one more thing - there are merchandise tag sensors in department stores. The people who are at the entrances/exits verify that you've paid for what you're carrying out if you set off the sensor. They aren't there to catch thieves - and if you've ever been to Wal-Mart, you should realize that the seniors that they have on that beat are not going to tackle a shoplifter, they'd probably break a hip...