The problem is that there's no way a vendor can guarantee it. They received corals from their suppliers and those corals may already have tiny aiptasia hidden inside the corals. The vendor can put in all the shrimp/fish/nudibranch they want but it's impossible to guarantee that those aiptasia will not be transferred to your tank when you buy their frags. I am not talking about visible aiptasia on the frags. Tiny baby aiptasia can be hiding in coral tiny cracks on the frag plug or even within skeletal structure of corals. They may take 1-3 weeks to become visible. How is vendor suppose to catch those besides quarantining their shipments for months? Some vendors are better than others at catching them / aquaculturing their own frags but aiptasia is very hard to completely eliminate. They can latch on inside pipes, overflows, sumps, etc where copperband, nudibranch, peppermint shrimp can't reach.
All I am saying is expecting 100% guarantee that your frag from a vendor is aiptasia free is not realistic.