I'm curious if trying on multiple shoes before purchasing is considered rude or inappropriate in the US
TLDR: You behavior was perfectly fine and appropriate. You looked in good faith for shoes that fit you, but found nothing. Your expectations of U.S. service level might have been too optimistic, but it's OK. The surly sales clerks were rude, but lousy customer service is becoming the norm in U.S. retail.
Background: Shopping malls and "brick and mortar" stores have been in decline for decades because consumers have been switching to ordering online and having their purchases delivered. Many stores have been closing. Few new ones open. Almost everybody who works in retail, especially on the store floor, feels disgruntled. In other countries, you might be used to being able to ask a salesperson for some expert advice about a product, or, at least, where something is on display. You might expect naively shopping to be fun! In the U.S... not so much, except possibly a few high-end stores that charge more for the same goods, but maybe a "nicer" shopping experience. Rather, the salespeople's duties are to deter shoplifters, not very effectively; and to "upsell". "Buying a fishing rod? Let me sell you some overpriced tackle too." "Buy this useless service contract, or the gadget you'te buying will turn into useless junk in weeks". The cheap manipulation you encountered, "boo hoo, you hurt our feelings so much by not buying anything" is their standard tool, trying to guilt the customer into buying something. I'm glad that you didn't fall for it.
You see now why more and more U.S. shoppers prefer to figure out the styles and sizes they need and order everything to be delivered.