It is a good question, and before experiencing it myself I'm not sure I'd have understood either. However, I think it's probably a combination of the fact that it's been so hastily set up, and the guards seem very poorly trained. The conflicting information I got about their policies in screening pregnant women was also very annoying, because it meant that we started four mornings on an adversarial note because we needed to push back when we were brought to the side (again, my mom and them are incorrect, the metal detector is safe, but instead of just saying this they said we should "let them know", which only made my mom think she was right and made it awkward.) I honestly know less about the wand and would have preferred a pat down, but the man who got huffy got very huffy and I felt really cowed and embarrassed, even though I was just following directions I had been given elsewhere. A pat down was never offered.
It didn't ruin my vacation, but it was terrible show. It's clearly not truly random and if you're the type to be selected, either out of expediency (like my mom and I) or out of profiling, it does cast a negative tone over the start of your park days. There are ways they could randomize to at least avoid the expediency grabs, and they clearly are not utilizing those techniques. With fewer than a quarter of folks getting selected (at eyeball), the random chances of our selection rate (5/7) are extremely low. We were also disproportionately pulled aside for surveys, so I think the expediency bias is real, and clearly not a good thing to be factoring in when making random security checks. The inconvenience and annoyance of having to empty your pockets and hand over your stuff right after going through bag check and getting recombobulated is hard to describe. Again, not day ruining, but very bad show by relatively unfriendly staff for a process that I believe is ineffective.
Thanks for getting it! It's hard to describe the feeling.