They don't have to accept anything. There were at least a half dozen houses than randomly went off the market with no warning that didn't have public sale offers.
If you list at $450,000 and multiple offers come in above that, how do you choose which one to take if you're required to take "full list". Most people don't care who is buying their house (in regards to illegal discrimination) but when everything is above list price the sellers look at who has waived inspections, who has an appraisal gap (we had garunteed $15,000 over appraisal for our sale), etc. They want the easiest transaction.
Our relator said he has clients who have FHA funding and they are locked out of everything because prices in those ranges are sold to all-cash investors. We made an offer on a $475,000 house in Plymouth that ended up going to an all-cash investors with no inspection. Who the heck does that??
Our house was listed at $450,000 and we offered $1,000 over next best price up to $490,000. We didn't waive inspection but we did say we promised not to ask for any money to fix any issues found (but it did give us an "out" if we needed it). We said we'd cover any appraisal gap up to $15,000 and all closing costs. In MN the buyers pay that. We're selling our house in Iowa and I sure hope it's the same thing here.
Anyway, we got it for $463,000 and the inspection was decent so we're moving forward. Like I said in the OP, the sellers told us our letter was the difference-maker.