This comparison is flawed because there is not legal pickpocketing, but there is legal immigration.

If there was a legal pickpocketing, and someone claimed to only be opposed to illegal pickpocketing, then it would be reasonable to point out that unless they're lying about their intent a solution to preventing illegal pickpocketing would be to make it all legal.

The analogy falls apart because nobody argues that they are "only" opposed to illegal pickpicketing.

If people are opposed to any form of immigration, then they should just admit that, rather than pretend they're only opposed to illegal immigration.

a. Opposed to someone taking my money against my will and the law just because they want to, “for a better life”.

b. Not opposed to someone taking my money in exchange for goods or services I want.

a. Opposed to someone moving into my country against my will and the law just because they want to, “for a better life”.

b. Not opposed to someone moving into my country because I married them and want them here.

There’s a whole spectrum between a and b, but I think most people are against a.

Legal pickpocketing is taxes you’re opposed to, or wages being garnished.

In theory people who say they’re only against illegal immigration are saying they completely agree with all policies regarding legal immigration, now and maybe into the future. Likely not what these people actually believe because while possible it would be a silly position. They’re probably just saying it to try to find some common ground with very pro immigration people. Likely a fools errand.