The point of misplaced modifiers is that they make the parsing of the sentence ambiguous. You say that the participles modify the subjects of the sentences ("I and she"), but the problem is that those aren't the explicit subjects of the sentence.
Interpretation #1:
1. I found a hundred dollar bill on the sidewalk walking down the street.
→ I was walking down the street.
2. She was also attending to her email talking on the phone.
→ She was walking down the street.
That's all fine, and it makes sense. But the syntax of the sentence is such that the following interpretation is also possible—and it might make sense in a fantasy world where such events can actually occur.
Interpretation #2:
1. I found a hundred dollar bill on the sidewalk walking down the street.
→ The hundred dollar bill was walking down the street.
2. She was also attending to her email talking on the phone.
→ Her email was talking on the phone.
Aside from common sense, there is nothing syntactical that can let you determine who or what was walking down the street or who or what was talking on the phone.
When you have a misplaced modifier, the subject is ambiguous. In these cases, it's fairly obvious who the subjects are that are doing the walking and talking. But consider different sentences where it's not nearly as obvious:
1. I found Sam on the sidewalk walking down the street.
→ Who was walking down the street—you or Sam?
2. She was also attending to her fiance talking on the phone.
→ Who was talking on the phone—her or her fiance?
Sentences with misplaced modifiers are syntactical, but the subjects of the verbs can be ambiguous, leading to either humorous misinterpretation or outright confusion.
Note that these sentences can be have the misplaced modifiers removed, thereby removing any ambiguity or misinterpretation. This can be done either by simply inserting while or by rephrasing them entirely:
1. I found a hundred dollar bill on the sidewalk while walking down the street. OR
1. I was walking down the street when I found a hundred dollar bill on the sidewalk.
2. She was also attending to her email while talking on the phone. OR
2. She was talking on the phone and also attending to her email.