This isn't unheard of and is actually something all major retailers do. Best Buy, Amazon, Target, etc all do this and many actually have a third-party that uses algorithms to monitor return habits and places various types of flags on accounts in real-time. Some of those flags are informational, some require manual review, and some are hard stops on buying or returning something online. You most commonly hear about people getting hit with these bans by Best Buy and occasionally Amazon as both appear to have tighter threshold than others.
If you didn't do anything wrong (not assuming either way), yes it can feel unfair. However, people don't understand the amount of fraud, loss, etc that comes with online retail so they've had to resort to various countermeasures. Just like you might get the occasional bad purchase, they may flag the "innocent" buyer.
Use MR as an example. Each time a new product comes out, or like with iPhones multiple varieties come out, people always post about how they're going to buy some and return whichever they don't like. They have every right to do that. But, Apple also has every right to monitor for that and if a person does it often without having enough normal, non returned purchases, they can decide the cost isn't worth the customer. Just because you kept one doesn't mean the profit offsets the 3-4 you've returned over the past year or two.