I am a special education teacher, and this is normal. Once a student has been identified as qualifying for special education services, the school legally liable to provide those services unless the student ceases to qualify or the parents refuse them. They are not trying to pressure you either way; they are trying to cover their own liability, and there is a federally-mandated timeline within which they have to either provide services, determine that services are not necessary, or have documentation that the parents refused services. They are only required to provide those services within the provided preschool program - that is, in fact, the purpose of the preschool program, to provide a place in which the services can occur; by law, students must receive services in the "least restrictive setting" - that is, in a setting that as closely as possible resembles the setting in which non-identified peers would be educated, which caused schools which did not have preschool programs to start them, with no more than 1/2 of the students having identified learning difficulties (else, it would be a special ed room with some non-special ed students - so no more than 1/2 can be identified).
If he truly has a speech dysfluency, it could (but may not) have a negative effect on his reading, as students who have difficulties pronouncing sounds may not be hearing them properly, and may therefore have trouble with them in reading and spelling; on the other hand, it may be a fine motor issue with his tongue that will resolve itself with greater physical maturity. Or it could be a number of other things that I don't have enough information or expertise to determine - I specialize in learning disabilities, not speech production. Only you can decide whether or not you want to enroll him in the program - but the school is merely trying to cover themselves, not pressure you one way or another, no matter how it seems to you.