The difference I think, is that the purity spiral on the left encompasses the entire party. If your perspectives are too moderate you are shunned from the entire hemisphere of politics and often suffer a barage of name calling (e.g. bigot) from your own 'side'.
On the right, this is far less often the case. The right is significantly more tolerant of people who fall outside of purity definitions. For example, the majority of republicans are pro-life, but co-exist with roughly a third of republicans who are pro-choice.
In contrast, most Democrats will not tolerate a pro-life member under any circumstance.
