There are several reasons why bet365 accounts can be restricted, including matched betting, unreasonable betting behaviour, GamStop (self-exclusion), and exploitation of bonuses. These are the most common reasons for the restriction of accounts with bet365.
In general, it is not legal to have multiple accounts with the same sportsbook in most states. This is because the sportsbook wants to ensure that you are only betting with them, and not with other sportsbooks.