The timing tools take a conservative approach to timing analysis. If there is an unconstrained path crossing clock domains, the tool will assume a relationship between the clocks which may result in timing violations. This is to highlight to the user that this path should be constrained.
If you would like the tool to ignore the path, please apply a TIG constraint. Another option is to use a FROM-TO constraint with the DATAPATHONLY keyword to ignore the clock skew.
The aim of the timing analysis is to ensure that all paths in the design are covered by constraints.