There is a special mode in MySQL that is designed for inputting the invalid data in a database. For example, in order to input in INT-field 2147483647 instead of 20000000000 or to fill the database with non-existent dates and cut lines. Well, you never know what fro this mode might be useful.
This mode is called a "traditional mode"
Although, at first glance it seems crazy and such a behavior is a historical reason.
The engine of MyISAM was a former standard engine for MySQL recently; it does not support the database transactions. If one thing falls off from a set of queries, then the database loses its integrity. In comparison with such risk an entry into the database of incorrect values was lesser evil.