The readonly attribute of ww:textfield is not checked on its value, but only on its existence.

The simple/text.ftl contains this:

<#if parameters.readonly?exists>
 readonly="readonly"<#rt/>
</#if>

But the documentation on ww:textfield (http://wiki.opensymphony.com/display/WW/textfield) says this:
readonly false false Boolean Whether the input is readonly

The simple/text.ftl should be changed as follows: (cfr disabled attribute)

<#if parameters.readonly?default(false)>
 readonly="readonly"<#rt/>
</#if>