LTspice default rising/falling times of pulse edges
is this a bug?


No.


  Can there be a reason for this default settings


Yes. This is explained in this FAQ on the LTwiki website:

I have a pulse source in my schematic with zero transition times.  LTspice only shows slow transition times of 2ns.  What's going on here?

It explains:

LTspice automatically will use a default value for &lt;trise&gt; and &lt;tfall&gt; if these parameters are set to zero.  Default value: 10% of Ton or 10% of Tperiod-Ton whatever is smaller.  You must specify Trise and Tfall if you want a certain value.