To be fair, scalping is a legitimate way to trade, so long as you can do it profitably. I cannot speak about 10-second trades with large volume, but to each their own. Brokers do not generally like scalpers, for obvious reasons, but most still allow it.
If a prop firm wants to prohibit scalping or implement a 30-second rule or similar, then they probably know that they will lose some business to other firms, although some smaller props do have this rule.
It all comes back to wanting your cake and eating it too. If a broker or prop wants to provide better trading conditions to attract more business, then they have to deal with the good and bad that comes with that (or implement prohibitive rules that will result in less business).
If a prop firm wants to prohibit scalping or implement a 30-second rule or similar, then they probably know that they will lose some business to other firms, although some smaller props do have this rule.
It all comes back to wanting your cake and eating it too. If a broker or prop wants to provide better trading conditions to attract more business, then they have to deal with the good and bad that comes with that (or implement prohibitive rules that will result in less business).