Usually it isn't the store pushing this, but Square itself. They take a percentage of each transaction so they naturally want to make the charges as high as possible.
As someone who uses square at our business, you can indeed customize it a little bit. Though I've never seen the "talk to a cashier button." Ours says custom tip where someone can place a zero $$ tip as well.
But to the point of square taking a cut... The entire amount (including tip) has a percentage taken by square... So they do want you to have as big a transaction as possible because they make more money.