Was it "prudent" by Walmart to not allow me in the store with a MAGA hat?

The Colorado shooter was a rabid anti Trump nut job 

Yes, "it's a free country", meaning Walmart has the legal right to prevent you or anyone else from entering their property for nearly any reason they want.  Satisfied?

Legal opinion:  we don't have an attorney-client relationship with you, so we cannot possibly give you a legal opinion that has any actual value as such.  Legal INFORMATION is free, and worth every penny.

The store can implement policies like that so long as it is applied fairly; e.g. no Biden hats are not allowed either. I think you did the right thing. 

No, you're not going to keep wearing it in the store.  You are going to be banned from the store the next time you enter WalMart WHICH IS A PRIVATE BUSINESS AND IS ALLOWED TO PICK AND CHOOSE ITS CUSTOMERS.  Every day in WalMart?  I'd rather be shot.

Walmart is private property, there is no law (federal or state or even local) that says you can not be banned from property because of what you wear. You are not covered under any federal laws.

If they banned you because of sex, age, race, religion then you would have a federal civil rights violation case, but nothing covers clothing

They are uneasy about what happened, people are known to be copy cats, Walmart did have a shooting awhile ago, they have every right to try to make sure their customers, employees and property is safe, in fact that is the companies duty to make it happen

If they were getting complaints about certain attire, then they have the right to forbid it from being worn

Many night clubs have dress policies, upper class restaurants have dress codes, you do not follow them you are not allowed in

Perfectly legal  

Have MAGA tattooed on your forehead.  Then they can't ask you to take it off.

This seems like one of those made up stories that conservatives feel "could have happened" 

But, you put some effort into this, so I'll give you partial credit.  

Walmart is private property and they can refuse you entry for any reason if you refuse they will call the cops and they will escort you out. Businesses don't refuse entry on any basis though because they want customers. 

You have the legal right to wear clothing that expresses your opinion—and businesses have the right to refuse you entry because they disagree, fear it promotes violence, worry it offends other shoppers, etc.

Is a hat the hill you want to die on, figuratively?

If you wear the symbol of bigots and Chud Traitors, don't be surprised when people treat you like a bigoted Chud Traitor.

We're not calling you people Nazi's because we don't like you, over on the left. We're calling you that because a Large percentage of the right has embraced their swastika tattoos, and lately, isn't afraid of showing them off.

Just leave the hat in your car. I'm not surprised uncultured swine wear it indoors in your state, but typically, people with class take it off in the store.

Just gonna go ahead and guess here, but I'd bet money that the greeter knows you, because your Trump Chud a*s didn't want to wear a mask, either right?