If they participated, not only would they have to pay a lot of money to Eric's Master, but they were also bolstering his strength.
2 years was a short time, but also a long time. the disciples they were going to send might change their mind and decide to stay with him, and they would be just adding soldiers against themselves.
At the same time, the temptation of having another 5 High Heaven strong powerhouses in their lineup was just too big.
Their best choice would be to either make sure that the disciples wouldn't betray them or negotiate with Eric's Master to not keep them after.
Certainly, the cheapest and wisest choice for them was the first one, they had to make sure that the disciples they would send to him wouldn't turn sides.
The best way to do that was clearly making them slaves, or death soldiers of the family. They either stayed sane and become powerhouses of the family, or perished without a trace.
