Dotted and dotless i is a different issue, related to capitalization. Those are still Latin letters. Of course to support most European languages you have to be able to use ASCII and non-ASCII Latin letters together. That's not unique to Turkish.
Think of the Spanish word "cañon", for example. It's not four English letters and one Spanish letter, it's just five Latin letters (four of which are in ASCII).
There is a difference between ı and i in both cases. I don't know what you mean by 'related to capitalisation'
The reason I raise Turkish specifically is that the similarity between the characters presents a potential homograph for a phishing attack in non-Turkish domains. (e.g. mıcrosoft.com). Characters with apparent diacritics are less vulnerable (e.g. öracle.com).
Think of the Spanish word "cañon", for example. It's not four English letters and one Spanish letter, it's just five Latin letters (four of which are in ASCII).