Why ambiguous grammars are allowed for operator precedence parser?
An operator precedence parser usually has no ambiguity at all. However, the evaluation order is not determined by the parse tree, but by the operator precedences and associativities. 

As a practical example, the Swift language doesn't define operator precedence or associativity in the language. It doesn't even define operators, except that the scanner knows which sequences of letters could be operators. The actual operators and precedence / associativity are define by the programmer, or as part of the standard library. As a result, a+b*c>d/e produces a parse tree that just consists of the five operands and four potential operators. The parse is unique, but doesn't tell how the expression should be evaluated. 

The next phase checks that all the four potential operators are indeed operators. Then the priorities and associativity of the operators, as defined by the programmer or standard library, are examined. * and / have highest priority, + has lower, and > has even lower priority. If you had a-b-c, then the two operators have the same precedence, and both are left-associative, so the order is also clear. If you had two or more consecutive operators with same priority and different associativity, or with no associativity, that would be an error.

You couldn't define a grammar to handle operators, because their precedence and associativity are not even known when you start compiling.