Because to many people, the phrase “impose reasonable limits” is exactly equivalent to “we’re gonna take your guns, you lunatics.”
I believe that given the information in the link that Babale provided earlier, the blanket statement that “The ACLU does not believe in the individual right to bear arms” isn’t accurate.
The ACLU has said:
But the categories of people that federal law currently prohibits from possessing or purchasing a gun are overbroad, not reasonably related to the state’s interest in public safety, and raise significant equal protection and due process concerns. Any number of the categories, for example, require no proof of dangerousness, and they often serve to further bias. For example, the list of those barred includes: anyone convicted of a crime punishable by imprisonment for more than a year, whether or not the crime has any connection to violence; people with mental disabilities and many noncitizens who have not been shown to be dangerous in any way; and those who have used substances on the federal controlled substance list, including marijuana in states in which it is legal.
Other proposed gun regulations also raise civil liberties concerns. The proposal to ban individuals listed on the No-Fly List from purchasing weapons, for example, is constitutionally problematic, because that list lacks basic due process protections and its standards are unconstitutionally vague.
That doesn’t comport with the idea that the ACLU doesn’t believe individuals should have a right to bear arms. If they did, then they wouldn’t have concerns with any laws taking guns out of the hands of individuals, but they clearly do.
They have said:
In striking down Washington D.C.'s handgun ban by a 5-4 vote, the Supreme Court’s decision in D.C. v. Heller held for the first time that the Second Amendment protects an individual’s right to keep and bear arms, whether or not associated with a state militia. The ACLU disagrees with the Supreme Court’s conclusion about the nature of the right protected by the Second Amendment. However, particular federal or state laws on licensing, registration, prohibition, or other regulation of the manufacture, shipment, sale, purchase or possession of guns may raise civil liberties questions.
So they’ve said, “The ACLU disagrees with the Supreme Court’s conclusion about the nature of the right protected by the Second Amendment.” But that doesn’t seem to mean that they don’t believe in individuals having the right to bear arms, period. It seems more nuanced than that.
Anyone who isn’t 100% pro-put guns into the hands of everyone is unfriendly to gun owners in the eyes of someone who only sees everything through a pro-gun lense.
Any criticism, no matter how small, is enough to draw their undying and irrational hatred.