If a law is consistently and chronicallhy broken (and especially if the harm for breaking said law is minimal), at some point one must ask if the law is too incompatible with human behavior to justify enforcement or existence.
I don't think anyone "soft on illegal immigration" is against legal immigration. But they may very well be against the laws as they currently stand, because the laws are demonstrably shaped by racist intent; the national quota system used to decide people per year from each country where very much structured out of fear of a "browning" of America.
I'm much less sympathetic to people breaking U.S. immigration law because they want greater economic opportunity for themselves or their children.
That said, I can't argue back to first principles why my preferred balance of virtue maximization / vice minimization is objectively better than anyone else's. So if we're being completely honest, I can't advocate even my own position in good conscience.
One of my main reasons for opposing illegal immigration is that it's contrary to the rule of law. It subverts the democratic process, and makes suckers of anybody who does try to follow these laws. I'm actually a big fan of legal immigration.
But I rarely (never?) hear this distinction made in public statements by advocates of illegal immigration. At least in public, they seem to assume that anybody who opposes illegal immigration opposes all immigration, and even worse, does so out of ignorance or bigotry.
Assuming I'm not the only person with my particular views, that conflation is infuriating. I can only hope it's done unwittingly.