Brexit: Interactive Brokers vs Firstrade

How come Interactive Brokers (IB) needs a UK subsidiary to service EU customers, while other brokers like Firstrade don’t? Now that Brexit is about to happen, IB will open other subsidiaries in the EU and the investment guarantee goes down from $500k to $20k. Since Firstrade doesn’t operate through subsidiaries, it doesn’t appear to have this problem. Am I missing something? Why does IB resort to subsidiaries?