They offered the 'service' of being able to trade large nominal amounts at the last market price, without market impact, but had incredible levels of margin -- 50-1+ or something like that.
This meant that customers often hit margin limits, incurring additional 'commissions' to close out their positions.
Operationally, the bucket shop netted long and short positions, and either actually traded the residual or, occasionally, would use market orders (e.g. aggressive sells) to push the punters into margin calls, which they would then execute. They would then buy low and cover their earlier positions.
Jesse Livermore managed to beat them through astute technical analysis and through the installation of a direct telegraph line from the exchange, thus beating his 'brokers', who promptly kicked him out.
It matters when things go badly wrong, because with a Bucket Shop you're left penniless, it was all imaginary anyway. Whereas with a futures market even if the exchange blows up those are real contracts you can sell to somebody who wants the commodity you were trading in, at worst a mild inconvenience and a small haircut to your final profit.
In the US, securities brokers are very heavily regulated — see “regulation NMS”.