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You can demand that they allow you to leave or they can call a cop and
have you arrested and then searched, if I correctly recall the last
time I heard complaints about this procedure (on another newsgroup, a
year or two ago).

If they do that, they'd better be able to explain that they have
reasonable suspicion to believe you have stolen merchandise (surveillance
camera, eyewitness, etc); refusal to allow a search is does not constitute
reasonable suspicion to believe a crime has been committed. If they held me
until police arrived only because I refused to submit to a search I'd sue
them for false arrest.

I do not believe they are empowered to physically
restrain you until you get outside the building and then only on
reasonable suspicion and willingness to call a police officer.

That depends on state law, in most states you have to exit the store
to complete the crime. Here in Colorado, secreting merchandise on your
person completes the crime (they don't have to let you leave the store).

If I, again, recall correctly, the reason they do it is that some of
their checkout persons are dishonest and in league with 'shoppers' to
give them goods at very low prices. This is not _your_ problem, it's
the store's problem and you should not be inconvenienced to solve
their incorrect personnel hiring and retention problems.

The manager of the local Walmart Supercenter told me they suffer
more from employee theft than they do from shoplifting. Serves them right. I
think the local Walmart requires new employees demonstrate a lengthy arrest
record, drug addiction, and a surly attitude (plenty of good reasons to
abuse the customers).