There's some trickiness in definitions of these terms because they have various connotations and have also changed over time - I will try and use them in the common way that people see the terms today.
But even within the contemporary connotation of "liberal" I think there's a difference between someone who holds some liberal views or ideals and Liberalism. Liberalism is a pro-capitalist view ("Progressive" is varried, it can be what basically a democratic socialist in the US might call themselves, but in general it would be a kind of reformist), today in official politics it basically means austerity with maybe a little less vitriol. But Liberalism fundamentally is a view about how to best preserve and expand capitalism.
Regular people who hold liberal views however, can be our allies and I think they are generally the people who we want to try and convince away from Liberal arguments and explanations in favor of revolutionary ones. But people who call themselves liberal generally are not liberal because of a set ideological worldview that they are committed to, but because they want union protections, they want to not be totally at the mercy of capitalist demands, they don't want racism or homophobia, etc.
But the problem is the Liberalism supports a system that maintains and bolsters the exploitation and oppressions and lack of real democratic input that many working class people with "liberal positions" hope that Liberalism will help ease.
And yes, radicals are generally hostile to Liberalism because it supports exploitation, oppressions, and imperialism. We are sometimes more hostile to it than we are to conservatism because Liberal politics are often a barrier to more radical possibilities because they offer a non-threatening non-solution to the overlapping concerns of say, anti-racist workers and anti-racist activists. So in a practical way we are more in competition with the ideas of Liberalism than with the ideas of Conservatism because it's not like conservatives are going to come to a grassroots anti-police brutality coalition and then convince other workers that we will best organize by having a voter-turnout for Obama or vote in a new Mayor.
A lot of the knee-jerk frustration from the Left comes from "Liberal hypocrisy" of appealing to semi-popular wishes but supporting the system that creates the initial problems anyway. Like how can liberals claim to be anti-war but then vote for Obama, forget LBJ and WWI and WWII were all under Presidents who got the "anti-war" vote.
Finally, I think anecdotally, for some of us, we were drawn to revolutionary ideas out of a frustration with the limitations and hypocrisy of Liberalism and liberal politicians. So it's natural that we would get a little irked by being lumped in with ideas we consciously rejected.


Ponies' Commissariat for Magic & Friendship
