So you didn't notice that the assembly you removed had more parts on it than the assembly you installed. How would you react if you had paid a certified mechanic to perform this repair and had the same end result? Would you blame the parts house for selling the mechanic incomplete parts assemblies or blame the mechanic for not installing it correctly?
On the bottom of one of my Autozone receipts-"Warranty excludes damage caused by misuse, abuse, other faulty parts, improper installation or off road, commercial or marine use." If you installed it without the snap ring that would be considered 'improper installation'.
I would rather have a tool I don't need, than need a tool I don't have.