1. High paying legal immigration is tightly controlled because highly skilled people and high-paying employers don't dare to break the law.
2. Low-paying illegal immigration is poorly controlled because poor people and small employers don't care about immigration law.
Pro-ICE-abolition groups, for the most part, want essentially open borders for poor people and are ambivalent about high skilled legal immigrants. ICE, for its part, has become increasingly brutal in its enforcement of immigration law, and has repeatedly been caught employing white nationalists and doing otherwise gross things. Getting rid of an unpopular enforcement agency is considered easier to accomplish politically than to actually re-write immigration law.