sorry I didn't mentioned, that I have already set the log model of the database to simple. Even with that configuration the Log growths in this way. I tried the suggestion with divide the data into smaller Batches. I managed that with creating a SSIS Package. In the properties of the 'data flow Task - OLE DB destitnation' it is possible to divide the load into Batches of 10K size. After that the copy of the data into the InMem Table works, without any unnormal growths of the log.
The exciting question will be, how SQL Server will handle this in a Environment, where it isn't possible to set the recovery model to simple (Mirrored Databases / Logshipped Databases). ;-) I hope in the RTM will be a improvent of that behavior.