we are not worried - we have a competitive advantage
As an operator I think this is fine. The reasons I think it is fine are:
1- They aren't spamming you. SPAM is unsolicited commercial email. This is not unsolicited. If you read the terms you agreed to, you would know that you accepted receiving promos from them.
2- They provide a valuable service to the gaming community. If you are in the US, you will know how well deserved e-wallet companies earn their money. Who wouldn't want Neteller/MoneyBookers back into the US market?
3- It was done tastefully. They hardly ever send out promotional emails and when they do, they are clear and provide good unique bonuses of interest to their readers.
4- To say that they are 'stealing' players is a stretch at best. If a player signs up to your newsletter you can email them. If a player signs up their newsletter, they can email them.
Now, I will say something about your point of our customers leaving us. Do we want our customers to go to other places? Of course not. 
But we aren't going to pretend that our customers are going to stay with us for lack of knowing other places. Players know other places exist. So why will they stay with us? Because we know how to gain a competitive advantage through service, promotions and other strategical conversion and retention methods.
We know our players know other places exist, but we know our players know those other places won't give them the same warm fuzzy feeling they get here at Vegas Regal Casino. What we do to ensure that fuzzy feeling, is our secret sauce, and the thing that lets us rest at ease when seeing these mailers.
Kind Regards,
Nicolas Johnson
BetPhoenixCasino Affiliate Manager