Hmmm. Under that line of reasoning, you could substitute anything for "...check your receipt at the door..." and it would be okay.
What if it was:
Walmart and Best Buy perform a strip search at the door; you know that going in.
If you don’t like it, don’t go in.
Or even:
Walmart and Best Buy have thugs that slap you around before you leave; you know that going in.
If you don’t like it, don’t go in.
That wouldn't be okay, would it? Of course not. It's ludicrous to even entertain the notion.
The idea being that just because you are aware of store policy going in doesn't make the policy okay.
“Slippery slope” doesn’t apply here.
Not sure why so many are so eager to bring an end to this practice. It’s not an invasion of privacy. It’s not a “slippery slope” to strip searches and background checks. It’s an effort to stop shoplifting.