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fixLogPlots[gr_] := gr /. f : (FrameTicks -> _) :> (f /. _Thickness :> (## &[])) 

fixLogPlots[gr_] := gr /. f : (Ticks | FrameTicks -> _) :> (f /. _Thickness :> (## &[])) 

Here is universal fix which works for all types of log plots:

fixLogPlots[gr_] := gr /. f : (Charting`ScaledTicks | Charting`ScaledFrameTicks)[{Log, Exp}] :> (Part[#, ;; , ;; 3] &@*f) 

Here is universal fix for version 10 which works for all types of log plots:

fixLogPlots[gr_] := gr /. f : (Charting`ScaledTicks | Charting`ScaledFrameTicks)[{Log, Exp}] :> (Part[#, ;; , ;; 3] &@*f) 

UPDATE 2

And here is universal fix for versions 8 and 9:

fixLogPlots[gr_] := gr /. f : (FrameTicks -> _) :> (f /. _Thickness :> (## &[])) 

Reproduced in v.10.0.0 under Win7 x64. In v.8.0.4 only major logarithmic frame ticks change their thickness, but not minor ticks.

So this behavior reflects inconsistent implementation of Charting`ScaledTicks and Charting`ScaledFrameTicks which should NOT include styling into the tick specifications they generate. It may be called a bug.

Reproduced in v.10.0.0 under Win7 x64. In versions 8.0.4 and 9.0.1 the behavior differs in details but the bug is also present: only major logarithmic frame ticks change their thickness, but not minor ticks.

So this behavior reflects inconsistent implementation of Charting`ScaledTicks and Charting`ScaledFrameTicks which should NOT include styling into the tick specifications they generate. It is a bug.