If your plan to improve your own working conditions requires you to obstruct others from seeking employment that they’d be happy to have, then you have a bad plan, and you don’t have any moral high ground either. Anybody at any level in any company has an incentive to deny others the right to compete with them. This doesn’t make you a hero to the laborers. You do have a right to picket, you have no right at all to obstruct others in their own search for employment, or in accessing their place of work. Collective bargaining gives you the ability to voluntarily combine your bargaining power. It does not give you any right at all to involuntarily strip others of their own.
If you want to know why employees themselves might be anti-union, and get confused as to why anybody would want something that you so clearly see as being against their own interests, this is why. If you don't like the union, it's not as simple as "don't join the union". Because as you demonstrate here, any union organiser with any ambition is going to make their first priority after forming the union to be pushing out non-union members, and restricting their ability to seek employment outside the CBA, by any and all means available (even regardless of legality).