Unfortunately, this does not just happen with bucket shops but many other seemingly reputable brokers as well. Mostly not to protect the customer but their own asses as exchanges try to prevent anyone from taking advantage of fat fingers or dumb individuals. The only chance to get filled 4 or 5 percent above the market is when there are no other resting orders in between. Otherwise with each partial fill the spread is tightened and a new market established.
Especially with bucket shops.
While the IBKR puts a warning on buy limit orders that are 4% above the market, other bucket shops will not fill your orders 5% above the market.
Day & night.
,,Zero commisions"