I think the people who make beach towels should spend some time with the people who make bath towels. There really shouldn't be any difference between the two, should there? They're both towels. Their primary function is to dry people off, yet bath towels are soft and fluffy and beach towels are thin and rough. Almost like the Snuggles bear said "Yay!! I'll make the bath towels!! Yeeee!" And Gritty, the sandpaper spokesman said, "Beach towels? I got this."
Another difference is bath towels are often one solid color and beach towels are an eyesore of bright colors, usually advertising your likes. A bath towel tells people, "This shade of blue compliments the border in the bathroom" and a beach towel tells people, "I like The Six Million Dollar Man!!!"