Why wrestling fans can chant whatever they want!
Professional wrestling is an interesting 'sport'. There is no other form of entertainment where the fans have such a direct impact on the product.
In fact, fan participation is essential for the success of the product.
Case in point:
Let's compare the two videos posted above. Both are the returns of legends to the WWE after a long gap. Both take place in the last segment of an episode of Monday Night Raw. But you get goosebumps only during the latter. The obvious reason for this is the fan reaction that it got. With the former, you do not get a feeling of ecstasy. Just another regular person making his entrance.
A silent crowd can make a good match seem boring(Triple H vs Seth Rollins from Wrestlemania 33), wheareas a hyped up crowd can make a bad match seem epic(The Rock vs Hulk Hogan from Wrestlemania 18). Such is the power of a fan reaction. The reason why people like Ricochet do huge spots on a weekly basis is to garner this reaction.
The audience can make or break a superstar. If it weren't for the 'Die Rocky Die' chants, we would have been stuck with 'Rocky Maivia' instead of 'The Rock'. Here, we can say that the audience both broke and made a superstar or in this case, a megastar.
The point that I am trying to make is that when such an essential is treated as something that isn't allowed to do, then the whole idea of professional wrestling is changed. This is not ballet, where the audience is expected to silently admire and applaud when the performance is over. This is professional wrestling, where the audience is very much a part of the product. If the audience just sits there and silently spectate the action, the result would be a completely drained product(a.k.a the Jake Roberts return).
If the audience is chanting irrelevant stuff like 'We want beachball', it just means that they are not invested in the product enough to care for it. How many times have we heard 'We want beachball' in an NXT Takeover event? The audience in an NXT Takeover event does not have the time to chant irrelevant stuff or take over the show(no pun intended). They are busy getting invested in the actual wrestling being put on by the superstars. But can we say the same for a main roster show?
In a way, these kinds of chants can be viewed as a rejection of the product or wrestler being shown. Do we hear any 'CM Punk' chant during a Daniel Bryan or Kevin Owens promo? No! But 'Cm Punk' chants can be deafening during a Roman Reigns promo.
One more infamous chant that gets thrown at wrestlers is the 'WHAT' chant. This can be seen as a litmus test for superstars. People who beat this chant will never be thrown at again. For example, look at the video below of Alexa Bliss owning the 'WHAT' chants.
On the whole, wrestling fans have the freedom to say whatever they want during a wrestling event because it is their way of expressing what they feel about the product. If the product is interesting for them, they would not even bother hijacking the show because they would be busy enjoying the show.