这么说吧,
for后面一般是接名词的.常见的如for you,for me.
你说的for后面接ing的确没错,
动词接ing之后构成动名词,
所以for可以接ing这个动名词,如Thank you for helping us.
而在本句中help是作名词使用的,
你可以去查字典,help不仅是动词,也是名词.
举个例子区别一下,
ask somebody for help(名词),
ask somebody to help(动词) us.