First of objectification and sexualization are not interchangeable terms, or rather objectification is broader category than sexualization.
Objectification refers to situations when a person (or group of people) is made into an object or action aiming at making object from a person. A person, from agent, who is free to act, becomes an object, that is to be acted upon or used (note that person does not literally use personhood, will to act, and certain abilities, but becomes this in the narrations, in perception of others and position in society gets lower). This is also not one-time occurrence, but is meant to strip agency from the person (or group of people) permanently and stages them to be perceived by others as objects by others. This in some cases may be limited to some spheres of human activity, for example to sexual sphere. But it also prevents parity of participation in the society (for those, who are being objectified).
Now, sexualization is usually understood as objectification in sphere of sexuality. This is dangerous. Objects don't get to say what they wants. Object, previously usually woman, don't get to decide over themselves. Object is to be used by others. Things like consent do not get into play. In mainstream narration only men can be sexual agents, women can be only objects. Especially troubling is sexualisation of children. That is making children and young teenagers objects, that can be used sexually by others.
To Beyonce's enthusiast - they think, that Beyonce is picturing women as agents in sexual sphere, and furthermore, as agents of good sexuality. We have history of women being denied agency in sexual sphere (mostly Victorian narration on female sexuality), I'm sure what Beyonce intends to do is to reclaim it. We also have history of women being seen as agents of "evil" in the sexual sphere (we can call it medieval narration on female sexuality). There is sex positivist movement in the feminism, that aims to shape and strengthen narration of women being positive sexual agents. In feminist narration everyone, who wishes, can be sexual agent, regardless of gender.
It's better to be an agent, than an object, as agent decides for themselves and can also decide not to act. Agents has ability to say "no" wherever they like. By that premise, it's better to show woman as sexual and in position where she gets to decide over her actions, than to de-sexualize her, which would just strip her from ability to participate independently in this sphere of human activity.
That totally abstract from morality of sexual actions perform by woman-agent. Whether they are moral or good role model is of lesser importance. Empowering factor lays in fact, that woman is active, is in power over her own body to use it as she pleases. Also many feminist would find examples of women acting in immoral (regardless if it's sexual or not) way as empowering, by sheer fact, that those women are agents. Imagine witches or evil empress, their actions are evil, but at the same time they are often effective actions in public sphere, and that, while maybe not a good role model, is empowering.
To Beyonce's critics - vast majority of them do not care about objectification, however they are pissed, that woman in the video isn't made into their kind of object. They don't want to see women as sexual, especially not as sexual agents. They would like to place women on pedestal of purity (so called respectable woman). That however do not help women. This is not position of parity, deprives women of agency at least in sexual sphere, and isn't protecting women that well from abuse (there are men, who would get additional kicks from taking women from the figurative pedestal and destroying them.)
So why those guys don't want to see women as agents? They simply can't imagine sexual situation with two agents, so in their narrow minds if woman is on position of an agent, that means man is on position of an object. They simply feel endanger by this. There is also issue of "medieval" narration mentioned earlier.
To prevent that, they steal some vocabulary from emancipation movements, just because it will make them look concerned for good of women.
What I think about the video. I didn't like it. I know the idea and I basically support the idea, but in a way I see it this just don't execute the idea very well. But it is a matter of subjective perception perhaps.
The irony of all this, is that I need to place somebody, although temporary, in position of object, to explain what objectification means, and that by itself is a form of objectification.
Okay, that is just introduction to further discussion. I did not covered questions of body image and boundary between what is objectification and what is not (that, as Monkey King says, is blurry).